Method and apparatus for digital demodulation and further processing of signals obtained in the measurement of electrical bioimpedance or bioadmittance in an object

Abstract

Methods and apparatus for digital demodulation of signals obtained in the measurement of electrical bioimpedance or bioadmittance of an object. One example comprises: generating an excitation signal of known frequency content; applying the excitation signal to the object; sensing a response signal of the object; sampling and digitizing the response signal to acquire a digitized response signal representing the response signal with respect to frequency content, amplitude and phase; correlating, for each frequency f AC of the excitation signal applied, digitized samples of the response signal, with discrete values representing the excitation signal; calculating, using the correlated signals for each frequency f AC of the excitation signal applied, complex values for the bioimpedance Z(f AC ); providing, over time, a set of digital bioimpedance waveforms Z(f AC ,t)); separating the base bioimpedance Z 0 (f AC ), from the waveforms; and separating the changes of bioimpedance ΔZ(f AC ,t), from the waveforms.

1 . A method for digital demodulation of signals obtained in the measurement of electrical bioimpedance of a biological object, wherein the method comprises the following operations: 
 placing a first current electrode and a second current electrode in contact with the biological object;    placing a first voltage sensing electrode and a second voltage sensing electrode in contact with the biological object;    applying an output of an AC current source to the first current electrode and the second current electrode, to cause an AC current to flow through the biological object between the first current electrode and the second current electrode;    measuring a voltage across the first voltage sensing electrode and the second voltage sensing electrode, wherein the measured voltage is produced due to application of the output of the AC current source to the first current electrode and the second current electrode;    digitizing the measured voltage to produce Object Voltage Samples;    obtaining a real part of the bioimpedance of the biological object, by correlating the Object Voltage Samples with corresponding reference current samples; and    obtaining an imaginary part of the bioimpedance of the biological object, by correlating the Object Voltage Samples with corresponding reference current samples that are shifted in phase by −90 degrees.    
   
   
       2 . The method of  claim 1 , further comprising: 
 calculating a magnitude of the bioimpedance of the biological object, by calculating the square root of the sum of the square of the real part of the bioimpedance of the biological object and the square of the imaginary part of the bioimpedance of the biological object; and    calculating a phase of the bioimpedance of the biological object plus a measurement system phase shift, by calculating the arctan of the ratio of the imaginary part of the bioimpedance of the biological object to the real part of the bioimpedance of the biological object.    
   
   
       3 . The method of  claim 1:   wherein the biological object is an animal; and    wherein the operation of digitizing the measured voltage produces unfitted Object Voltage Samples, and wherein the operations further comprise fitting the unfitted Object Voltage Samples to discrete values of an ideal sinusoid to produce the Object Voltage Samples.    
   
   
       4 . A method for digital demodulation of signals obtained in the measurement of electrical bioimpedance of a biological object, wherein the method comprises the following operations: 
 placing a first current electrode and a second current electrode in contact with the biological object;    placing a first voltage sensing electrode and a second voltage sensing electrode in contact with the biological object;    applying an output of an AC current source to the first current electrode and the second current electrode, to cause an AC current to flow through the biological object between the first current electrode and the second current electrode;    measuring a voltage between the first voltage sensing electrode and the second voltage sensing electrode, wherein the measured voltage is produced due to application of the output of the AC current source to the first current electrode and the second current electrode;    digitizing the measured voltage to produce Object Voltage Samples;    producing a voltage that is directly proportional to, and in phase with, the output of the AC current source;    digitizing the voltage that is directly proportional to, and in phase with, the output of the AC current source, to produce Object Current Samples;    obtaining a real part of the bioimpedance of the biological object, by correlating the Object Voltage Samples with the Object Current Samples; and    obtaining an imaginary part of the bioimpedance of the biological object, by correlating the Object Voltage Samples with corresponding Object Current Samples that are shifted in phase by −90 degrees.    
   
   
       5 . The method of  claim 4 , further comprising: 
 calculating a magnitude of the bioimpedance of the biological object, by calculating the square root of the sum of the square of the real part of the bioimpedance of the biological object and the square of the imaginary part of the bioimpedance of the biological object; and    calculating a phase of the bioimpedance of the biological object plus a measurement system phase shift, by calculating the arctan of the ratio of the imaginary part of the bioimpedance of the biological object to the real part of the bioimpedance of the biological object.    
   
   
       6 . The method of  claim 4 , wherein the operation of digitizing the voltage that is directly proportional to, and in phase with, the output of the AC current source produces unfitted Object Current Samples, and wherein the operations further comprise fitting the unfitted Object Current Samples to discrete values of an ideal sinusoid to produce the Object Current Samples.  
   
   
       7 . The method of  claim 4 , wherein the operations further comprise: 
 providing, over time, a set of digital bioimpedance waveforms Z(f AC ,t));    separating a base bioimpedance Z 0 (f AC ) from the waveforms;    separating changes of bioimpedance ΔZ(f AC ,t) from the waveforms;    determining a rate of change of the bioimpedance dZ(f AC ,t)/dt; and    recording a temporal course of the base bioimpedance Z 0 (f AC ) and of the rate of change of the bioimpedance dZ(f AC ,t)/dt.    
   
   
       8 . A method for digital demodulation of signals obtained in the measurement of electrical bioimpedance of an object, wherein the method comprises the following operations: 
 placing a first current electrode and a second current electrode in contact with the object;    placing a first voltage sensing electrode and a second voltage sensing electrode in contact with the object;    applying the output of an AC current source to the first current electrode and the second current electrode, to cause an AC current to flow through the object between the first current electrode and the second current electrode;    measuring a voltage between the first voltage sensing electrode and the second voltage sensing electrode, wherein the measured voltage is produced due to application of the output of the AC current source to the first current electrode and the second current electrode;    digitizing the measured voltage to produce Object Voltage Samples;    calculating an in-phase portion of the AC current through the object, by correlating the Object Current Samples with corresponding discrete values of a unity sine waveform; and    calculating a quadrature portion of the AC current through the object, by correlating the Object Current Samples with corresponding discrete values of a unity cosine waveform.    
   
   
       9 . The method of  claim 8 , further comprising: 
 calculating a AC current magnitude through the object, by calculating the square root of the sum the squared in-phase portion of the AC current through the object and the squared quadrature portion of the AC current through the object;    calculating a phase of the AC current through the object plus measurement system current phase shift, by calculating the arctan of the ratio of the quadrature portion of the AC current through the object and the in-phase portion of the AC current through the object;    calculating an in-phase portion of the voltage, by correlating the Object Voltage Samples with corresponding discrete values of a unity sine waveform; and    calculating a quadrature portion of the voltage, by correlating the Object Voltage Samples with corresponding discrete values of a unity cosine waveform.    
   
   
       10 . The method of  claim 9 , further comprising: 
 calculating a voltage magnitude across the object, by calculating the square root of the sum the squared in-phase portion of the voltage and the squared quadrature portion of the voltage;    calculating a phase of the voltage across the object plus measurement system voltage phase shift, by calculating the arctan of the ratio of the quadrature portion of the voltage and the in-phase portion of the voltage;    calculating a magnitude of the bioimpedance of the object, by calculating the ratio of the voltage magnitude across the object to the AC current magnitude through the object; and    calculating a phase of the bioimpedance of the object plus a measurement system phase shift, by subtracting the phase of the AC current through the object plus measurement system current phase shift, from the phase of the voltage across the object plus measurement system voltage phase shift.    
   
   
       11 . The method of  claim 10:   wherein the object is a human being; and    wherein the AC current has a plurality of frequencies, and wherein the applying, measuring, digitizing, and calculating operations of claims  8 ,  9  and  10  are repeated for each frequency of the AC current.    
   
   
       12 . A method for digital demodulation of signals obtained in the measurement of electrical bioimpedance of a biological object, wherein the method comprises the following operations: 
 applying the output of an AC current source to a calibration impedance, to cause an AC current to flow through the calibration impedance;    measuring a voltage across the calibration impedance, wherein the measured voltage is produced due to application of the AC current source to the calibration impedance;    digitizing the voltage measured across the calibration impedance to produce Calibration Voltage Samples;    calculating a value proportional to an in-phase portion of the calibration impedance uncorrected for measurement system phase shift, by correlating the Calibration Voltage Samples with corresponding discrete values of a unity sine waveform; and    calculating a value proportional to a quadrature portion of the calibration impedance uncorrected for measurement system phase shift, by correlating the Calibration Voltage Samples with corresponding discrete values of a unity cosine waveform.    
   
   
       13 . The method of  claim 12 , further comprising: 
 calculating a magnitude of an equivalent to the calibration impedance, by calculating the square root of the sum of the squared in-phase portion of the calibration impedance and the squared quadrature portion of the calibration impedance;    calculating a phase of the calibration impedance including measurement system phase shift, by calculating the arctan of the ratio of the quadrature portion of the calibration impedance and the in-phase portion of the calibration impedance;    placing a first current electrode and a second current electrode in contact with the biological object;    placing a first voltage sensing electrode and a second voltage sensing electrode in contact with the biological object;    applying the output of the AC current source to the first current electrode and the second current electrode, to cause an AC current to flow through the biological object between the first current electrode and the second current electrode;    measuring a voltage between the first voltage sensing electrode and the second voltage sensing electrode, wherein the measured voltage is produced due to application of the output of the AC current source to the first current electrode and the second current electrode;    digitizing the voltage measured between the first voltage sensing electrode and the second voltage sensing electrode to produce Object Voltage Samples;    calculating a value proportional to an in-phase portion of the biological object bioimpedance, by correlating the Object Voltage Samples with corresponding discrete values of a unity sine waveform; and    calculating a value proportional to a quadrature portion of the biological object bioimpedance, by correlating the Object Voltage Samples with corresponding discrete values of a unity cosine waveform.    
   
   
       14 . The method of  claim 13 , further comprising: 
 calculating a magnitude of an equivalent to the biological object bioimpedance, by calculating the square root of the sum of the squared in-phase portion of the biological object bioimpedance and the squared quadrature portion of the biological object bioimpedance;    calculating a phase of the biological object bioimpedance plus measurement system phase shift, by calculating the arctan of the ratio of the quadrature portion of the biological object bioimpedance and the in-phase portion of the biological object bioimpedance; and    calculating the magnitude of the biological object bioimpedance, by determining the ratio of a known calibration impedance magnitude to the calibration impedance magnitude equivalent, multiplied by the magnitude equivalent of the biological object bioimpedance.    
   
   
       15 . The method of  claim 14:   wherein the operation of digitizing the voltage measured across the calibration impedance produces unfitted Calibration Voltage Samples, and wherein the operations further comprise fitting the unfitted Calibration Voltage Samples to discrete values of an ideal sinusoid to produce the Calibration Voltage Samples; and    wherein the operation of digitizing the voltage measured between the first voltage sensing electrode and the second voltage sensing electrode produces unfitted Object Voltage Samples, and wherein the operations further comprise fitting the unfitted Object Voltage Samples to discrete values of an ideal sinusoid to produce the Object Voltage Samples.    
   
   
       16 . The method of  claim 14 , wherein the operations further comprise calculating the phase of the biological object bioimpedance by determining the difference between the phase of the calibration impedance, and the phase of the biological object bioimpedance plus measurement system phase shift.  
   
   
       17 . A method for digital demodulation of signals obtained in the measurement of electrical bioimpedance of a biological object, wherein the method comprises the following operations: 
 applying the output of an AC current source to a calibration impedance, to cause an AC current to flow through the calibration impedance;    producing a voltage that is directly proportional to, and in phase with, the output of the AC current source;    digitizing the voltage that is directly proportional to, and in phase with, the output of the AC current source, to produce Calibration Current Samples;    measuring a voltage across the calibration impedance, wherein the measured voltage is produced due to application of the AC current source to the calibration impedance;    digitizing the voltage measured across the calibration impedance to produce Calibration Voltage Samples;    obtaining a value proportional to the real part of the calibration impedance, by correlating the Calibration Voltage Samples with the Calibration Current Samples;    obtaining a value proportional to the imaginary part of the calibration impedance, by correlating the Calibration Voltage Samples with corresponding Calibration Current Samples that are shifted in phase by −90 degrees;    calculating a calibration impedance magnitude equivalent, by calculating the square root of the sum of the square of the real part of the calibration impedance and the square of the imaginary part of the calibration impedance; and    calculating a calibration impedance phase plus a measurement system phase shift, by calculating the arctan of the ratio of the imaginary part of the calibration impedance to the real part of the calibration impedance.    
   
   
       18 . The method of  claim 17 , further comprising: 
 placing a first current electrode and a second current electrode in contact with the biological object;    placing a first voltage sensing electrode and a second voltage sensing electrode in contact with the biological object;    applying the output of the AC current source to the first current electrode and the second current electrode, to cause an AC current to flow through the biological object between the first current electrode and the second current electrode;    producing a voltage that is directly proportional to, and in phase with, the output of the AC current source that is applied to the first current electrode and the second current electrode;    digitizing the voltage that is directly proportional, and in phase with, the output of the AC current source, to produce Object Current Samples;    measuring a voltage between the first voltage sensing electrode and the second voltage sensing electrode, wherein the measured voltage is produced due to application of the output of the AC current source to the first current electrode and the second current electrode; and    digitizing the voltage measured between the first voltage sensing electrode and the second voltage sensing electrode to produce Object Voltage Samples.    
   
   
       19 . The method of  claim 18 , further comprising: 
 obtaining a real part of the bioimpedance of the biological object, uncorrected for measurement system phase shift, by correlating the Object Voltage Samples with the Object Current Samples;    obtaining an imaginary part of the bioimpedance of the biological object, uncorrected for measurement system phase shift, by correlating the Object Voltage Samples with corresponding Object Current Samples that are shifted in phase by −90 degrees;    calculating a biological object bioimpedance magnitude equivalent, by calculating the square root of the sum of the square of the real part of the bioimpedance of the biological object and the square of the imaginary part of the bioimpedance of the biological object;    calculating a phase of the bioimpedance of the biological object uncorrected for measurement system phase shift, by calculating the arctan of the ratio of the imaginary part of the bioimpedance of the biological object to the real part of the bioimpedance of the biological object; and    calculating a magnitude of the biological object bioimpedance, by determining the ratio of a known calibration impedance magnitude and the calibration impedance magnitude equivalent, multiplied by the magnitude equivalent of the biological object bioimpedance.    
   
   
       20 . The method of  claim 19 , wherein the phase of the biological object bioimpedance is the difference between the uncorrected phase of the biological object bioimpedance, and the phase of the calibration impedance.  
   
   
       21 . The method of  claim 19:   wherein, when the output of the AC current source is applied to the calibration impedance, the operation of digitizing the voltage that is directly proportional and in phase with the output of the AC current source produces unfitted Calibration Current Samples, and wherein the operations further comprise fitting the unfitted Calibration Current Samples to discrete values of an ideal sinusoid to produce the Calibration Current Samples;    wherein the operation of digitizing the voltage measured across the calibration impedance produces unfitted Calibration Voltage Samples, and wherein the operations further comprise filling the unfitted Calibration Voltage Samples to discrete values of an ideal sinusoid to produce the Calibration Voltage Samples;    wherein, when the output of the AC current source is applied to the first current electrode and the second current electrode, the operation of digitizing the voltage that is directly proportional and in phase with the output of the AC current source produces unfitted Object Current Samples, and wherein the operations further comprise fitting the unfitted Object Current Samples to discrete values of an ideal sinusoid to produce the Object Current Samples; and    wherein the operation of digitizing the voltage measured between the first voltage sensing electrode and the second voltage sensing electrode, produces unfitted Object Voltage Samples, and wherein the operations further comprise fitting the unfitted Object Voltage Samples to discrete values of an ideal sinusoid to produce the Object Voltage Samples.    
   
   
       22 . A method for digital demodulation of signals obtained in the measurement of electrical bioimpedance of an object, wherein the method comprises the following operations: 
 applying the output of an AC current source to a calibration impedance, to cause an AC current to flow through the calibration impedance;    producing a voltage that is directly proportional to, and in phase with, the output of the AC current source;    digitizing the voltage that is directly proportional to, and in phase with, the output of the AC current source, to produce Calibration Current Samples;    measuring a voltage across the calibration impedance, wherein the measured voltage is produced due to application of the output of the AC current source to the calibration impedance;    digitizing the voltage measured across the calibration impedance to produce Calibration Voltage Samples;    calculating a value proportional to an in-phase portion of the calibration current, by correlating the Calibration Current Samples with corresponding discrete values of a unity sine waveform; and    calculating a value proportional to a quadrature portion of the calibration current, by correlating the Calibration Current Samples with corresponding discrete values of a unity cosine waveform.    
   
   
       23 . The method of  claim 22 , further comprising: 
 calculating a magnitude of an equivalent to the current through the calibration impedance, by calculating the square root of the sum of the squared in-phase portion of the calibration current and the squared quadrature portion of the calibration current; and    calculating a phase of the calibration current including measurement system phase shift, by calculating the arctan of the ratio of the quadrature portion of the calibration current and the in-phase portion of the calibration current.    
   
   
       24 . The method of  claim 23 , further comprising: 
 calculating a value proportional to an in-phase portion of the calibration voltage uncorrected for measurement system phase shift, by correlating the Calibration Voltage Samples with corresponding discrete values of a unity sine waveform;    calculating a value proportional to a quadrature portion of the calibration voltage uncorrected for measurement system phase shift, by correlating the Calibration Voltage Samples with corresponding discrete values of a unity cosine waveform;    calculating a magnitude of an equivalent to the voltage across the calibration impedance, by calculating the square root of the sum of the square of the value proportional to the in-phase portion of the calibration voltage and the square of the value proportional to the quadrature portion of the calibration voltage;    calculating a phase of the calibration voltage including measurement system phase shift, by calculating the arctan of the ratio of the value proportional to the quadrature portion of the calibration voltage and the value proportional to the in-phase portion of the calibration voltage; and    calculating an equivalent to the calibration impedance magnitude by calculating the ratio of the voltage magnitude equivalent across the calibration impedance and the magnitude of the current magnitude equivalent through the calibration impedance.    
   
   
       25 . The method of  claim 24 , further comprising: 
 placing a first current electrode and a second current electrode in contact with the object;    placing a first voltage sensing electrode and a second voltage sensing electrode in contact with the object;    applying the output of the AC current source to the first current electrode and the second current electrode, to cause an AC current to flow through the object between the first current electrode and the second current electrode;    producing a voltage that is directly proportional to, and in phase with, the output of the AC current source that is applied to the first current electrode and the second current electrode;    digitizing the voltage that is directly proportional to, and in phase with, the output of the AC current source, to produce Object Current Samples;    calculating an in-phase portion of the AC current through the object uncorrected for measurement system phase shift, by correlating the Object Current Samples with corresponding discrete values of a unity sine waveform;    calculating a quadrature portion of the AC current through the object uncorrected for measurement system phase shift, by correlating the Object Current Samples with corresponding discrete values of a unity cosine waveform;    calculating an equivalent of the object current magnitude through the object, by calculating the square root of the sum the squared in-phase portion of the current and the squared quadrature portion of the current through the object; and    calculating a phase of the object current including measurement system phase shift, by calculating the arctan of the ratio of the quadrature portion of the current and the in-phase portion of the current through the object.    
   
   
       26 . The method of  claim 25 , further comprising: 
 measuring a voltage between the first voltage sensing electrode and the second voltage sensing electrode, wherein the measured voltage is produced due to application of the output of the AC current source to the first current electrode and the second current electrode;    digitizing the voltage measured between the first voltage sensing electrode and the second voltage sensing electrode to produce Object Voltage Samples;    calculating an in-phase portion of the object voltage uncorrected for measurement system phase shift, by correlating the Object Voltage Samples with corresponding discrete values of a unity sine waveform;    calculating a quadrature portion of the object voltage uncorrected for measurement system phase shift, by correlating the Object Voltage Samples with corresponding discrete values of a unity cosine waveform;    calculating an equivalent voltage magnitude across the object, by calculating the square root of the sum of the squared in-phase portion of the uncorrected object voltage and the squared quadrature portion of the uncorrected object voltage;    calculating a phase of the voltage across the object plus measurement system voltage phase shift, by calculating the arctan of the ratio of the quadrature portion of the object voltage and the in-phase portion of the object voltage;    calculating the magnitude equivalent of the bioimpedance of the object, by calculating the ratio of the voltage equivalent magnitude across the object to the current magnitude equivalent through the object, multiplied by the cosine of the phase shift between the voltage across the object bioimpedance and the current through the object bioimpedance; and    calculating the magnitude of the object bioimpedance by calculating the ratio of a previously known calibration impedance magnitude, to the magnitude equivalent of the calibration impedance, multiplied by the magnitude equivalent of the object bioimpedance.    
   
   
       27 . The method of  claim 26 , wherein the operations further comprise: 
 providing, over time, a set of digital bioimpedance waveforms Z(f AC ,t));    separating a base bioimpedance Z 0 (f AC ) from the waveforms;    separating changes of bioimpedance ΔZ(f AC ,t) from the waveforms;    determining a rate of change of the bioimpedance dZ(f AC ,t)/dt; and    recording a temporal course of the base bioimpedance Z 0 (f AC ) and of the rate of change of the bioimpedance dZ(f AC ,t)/dt.    
   
   
       28 . The method of  claim 26:   wherein the AC current has a plurality of frequencies; and    wherein the measurement system phase shift is the difference between the phase of the calibration voltage and the phase of the applied AC voltage.    
   
   
       29 . A method for digital demodulation of signals obtained in the measurement of electrical bioimpedance of an object, wherein the method comprises the following operations: 
 generating an excitation signal of known frequency content;    applying the excitation signal to the object;    sensing a response signal of the object;    sampling and digitizing the response signal to acquire a digitized response signal representing the response signal with respect to frequency content, amplitude and phase;    correlating, for each frequency f AC  of the excitation signal applied, digitized samples of the digitized response signal, with discrete values representing the excitation signal;    calculating, using the correlated signals for each frequency f AC  of the excitation signal applied, complex values for the bioimpedance Z(f AC );    providing, over time, a set of digital bioimpedance waveforms Z(f AC ,t));    separating the base bioimpedance Z 0 (f AC ) from the waveforms; and    separating the changes of bioimpedance ΔZ(f AC ,t) from the waveforms.    
   
   
       30 . The method of  claim 29 , wherein the operations further comprise: 
 determining a rate of change of the bioimpedance dZ(f AC ,t)/dt; and    recording a temporal course of the base bioimpedance Z 0 (f AC ) and of the rate of change of the bioimpedance dZ(f AC ,t)/dt.    
   
   
       31 . The method of  claim 30 , wherein, in the correlating operation, the discrete values representing the excitation signal are discrete values of a sinusoidal reference signal for the excitation signal.  
   
   
       32 . The method of  claim 29 , wherein the operations further comprise: 
 determining a rate of change of the changes in bioimpedance d(ΔZ(f AC ,t))/dt; and    recording a temporal course of the base bioimpedance Z 0 (f AC ) and of the changes of the bioimpedance ΔZ(f AC ,t).    
   
   
       33 . The method of  claim 32 , wherein, in the correlating operation, the discrete values representing the excitation signal are discrete values of a sinusoidal reference signal for the excitation signal.  
   
   
       34 . The method of  claim 29 , wherein the excitation signal is a sinusoidal signal of a known single frequency f AC .  
   
   
       35 . The method of  claim 29 , wherein the excitation signal has an amplitude and a phase that are substantially constant over time.  
   
   
       36 . The method of  claim 35 , wherein the excitation signal has an alternating current (AC) amplitude in a range of about 0.01 mA to about 5 mA.  
   
   
       37 . The method of  claim 29 , wherein the operations further comprise applying the excitation signal to a calibration impedance.  
   
   
       38 . The method of  claim 37 , wherein the calibration impedance is an ohmic resistor.  
   
   
       39 . The method of  claim 29 , wherein the operation of generating the excitation signal comprises: 
 reading discrete values of at least one sinusoidal waveform, stored in an addressable look-up table;    converting the discrete values of the at least one sinusoidal waveform into analog signals having a desired frequency content, amplitude, and phase.    
   
   
       40 . The method of  claim 29 , wherein the operation of generating the excitation signal comprises: 
 employing time-controlled direct digital synthesizing (DDS); and    driving an excitation source.    
   
   
       41 . The method of  claim 29 , wherein the excitation signal contains frequencies in a range of about 1 kHz to about 1 MHz.  
   
   
       42 . The method of  claim 29 , wherein the excitation signal contains frequencies in a range of about 10 kHz to about 200 kHz.  
   
   
       43 . The method of  claim 29 , wherein the response signal is sampled at a rate that is in a range from about 4 to about 20 times the highest frequency of the excitation signal.  
   
   
       44 . The method of  claim 29 , wherein the response signal is sampled at a rate that is about 10 times the highest frequency of the excitation signal.  
   
   
       45 . The method of  claim 29 , wherein the discrete values representing the excitation signal are produced at a sampling rate that is in a range from about 4 to about 20 times the highest frequency of the excitation signal.  
   
   
       46 . The method of  claim 29 , wherein the discrete values representing the excitation signal are produced at a sampling rate that is about 10 times the highest frequency of the excitation signal.  
   
   
       47 . The method of  claim 29 , wherein the operation of separating the base bioimpedance Z 0 (f AC ) from the waveforms comprises inputting the set of digital bioimpedance waveforms Z(f AC ,t) to a low pass filter to obtain the base bioimpedance Z 0 (f AC )  
   
   
       48 . The method of  claim 29 , wherein the operation of separating the changes of bioimpedance ΔZ(f AC ,t) from the waveforms comprises inputting the set of digital bioimpedance waveforms Z(f AC ,t) to a high pass filter to obtain the changes of bioimpedance ΔZ(f AC ,t).  
   
   
       49 . The method of  claim 29 , wherein the operations further comprise inputting the set of digital bioimpedance waveforms Z(f AC ,t) to a differentiator to obtain a rate of change of the changes in bioimpedance d(ΔZ(f AC ,t))/dt.  
   
   
       50 . The method of  claim 29 , wherein the operations further comprise inputting the set of digital bioimpedance waveforms Z(f AC ,t) to a differentiator to obtain a rate of change in the bioimpedance waveforms dZ(f AC ,t)/dt.  
   
   
       51 . The method of  claim 29 , wherein separate correlation processes are used for determining, respectively, an in-phase portion Re(Z(f AC ,t)) and a quadrature portion Im(Z(f AC ,t)) of the bioimpedance of the object.  
   
   
       52 . A method for digital demodulation of signals obtained in the measurement of electrical bioadmittance of an object, wherein the method comprises the following operations: 
 generating an excitation signal of known frequency content;    applying the excitation signal to the object;    sensing a response signal of the object;    sampling and digitizing the response signal to acquire a digitized response signal representing the response signal with respect to frequency content, amplitude and phase;    correlating, for each frequency f AC  of the excitation signal applied, digitized samples of the digitized response signal, with discrete values representing the excitation signal;    calculating, using the correlated signals for each frequency f AC  of the excitation signal applied, complex values for the bioadmittance Y(f AC );    providing, over time, a set of digital bioadmittance waveforms Y(f AC ,t);    separating the base bioadmittance Y 0 (f AC ) from the waveforms; and    separating the changes of bioadmittance ΔY(f AC ,t) from the waveforms.    
   
   
       53 . The method of  claim 52 , wherein the operations further comprise: 
 determining a rate of change of the bioadmittance dY(f AC ,t)/dt; and    recording a temporal course of the base bioadmittance Y 0 (f AC ) and of the rate of change of the bioadmittance dY(f AC ,t)/dt.    
   
   
       54 . The method of  claim 52 , wherein the operations further comprise: 
 determining a rate of change of the changes in bioadmittance d(ΔY(f AC ,t))/dt; and    recording a temporal course of the base bioadmittance Y 0 (f AC ) and of the changes of bioadmittance ΔY(f AC ,t).    
   
   
       55 . The method of  claim 52 , wherein the operation of separating the base bioadmittance Y 0 (f AC ) from the waveforms comprises inputting the set of digital bioadmittance waveforms Y(f AC ,t) to a low pass filter to obtain the base bioadmittance Y 0 (f AC )  
   
   
       56 . The method of  claim 52 , wherein the operation of separating the changes in bioadmittance ΔY(f AC ,t) from the waveforms comprises inputting the set of digital bioadmittance waveforms Y(f AC ,t) to a high pass filter to obtain the changes of bioadmittance ΔY(f AC ,t).  
   
   
       57 . The method of  claim 52 , wherein the operations further comprise inputting the set of digital bioadmittance waveforms Y(f AC ,t) to a differentiator to obtain a rate of change of the changes in bioadmittance d(ΔY(f AC ,t))/dt.  
   
   
       58 . The method of  claim 52 , wherein the operations further comprise inputting the set of digital bioadmittance waveforms Y(f AC ,t) to a differentiator to obtain a rate of the change in the bioadmittance waveforms dY(f AC ,t)/dt.  
   
   
       59 . A method for digital demodulation of signals obtained in the measurement of electrical bioimpedance of an object, wherein the method comprises the following operations: 
 applying a calibration excitation signal to a calibration impedance;    measuring, sampling and digitizing a signal representing the calibration excitation signal to acquire calibration Excitation Signal Samples;    measuring, sampling and digitizing a calibration response signal across the calibration impedance to acquire calibration Response Signal Samples;    for each frequency f AC  of the calibration excitation signal applied to the calibration impedance, correlating the calibration Excitation Signal Samples with discrete values of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the calibration excitation signal related to the ideal sine waveform as a reference sine; and    for each frequency f AC  of the calibration excitation signal applied to the calibration impedance, correlating the calibration Excitation Signal Samples with discrete values of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the calibration excitation signal wherein the AC current has a plurality of frequencies.    
   
   
       60 . The method of  claim 59 , further comprising: 
 correlating the calibration Response Signal Samples with discrete values of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the calibration response signal; and    correlating the calibration Response Signal Samples with discrete values of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the calibration response signal.    
   
   
       61 . The method of  claim 60 , further comprising: 
 calculating an equivalent for a magnitude and a phase of the calibration excitation signal;    calculating an equivalent for a magnitude and a phase of the calibration response signal;    calculating an equivalent for a magnitude of the calibration impedance;    calculating a system phase;    applying an object excitation signal to the object after the operation of calculating the system phase;    measuring, sampling and digitizing a signal representing the object excitation signal to acquire object Excitation Signal Samples;    measuring, sampling and digitizing the object response signal across the bioimpedance of the object, wherein the samples obtained from sampling the object response signal are called object Response Signal Samples;    for each frequency f AC  of the object excitation signal applied, correlating the object Excitation Signal Samples with discrete values of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the object excitation signal related to the ideal sine waveform;    for each frequency f AC  of the excitation signal applied, correlating the object Excitation Signal Samples with discrete values of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the object excitation signal;    correlating the object Response Signal Samples with discrete values of another ideal sine waveform in order to obtain a value proportional to an in-phase portion of the object response signal;    correlating the object Response Signal Samples with discrete values of another ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the object response signal;    calculating an equivalent for the magnitude and a phase of the object excitation signal;    calculating an equivalent for the magnitude and a phase of the object response signal;    calculating an equivalent for the magnitude and a phase of the bioimpedance of the object;    calculating a magnitude of the bioimpedance Z(f AC ,t) of the object; and    calculating an in-phase portion Re(Z(f AC ,t)) and a quadrature portion Im(Z(f AC ,t)) of the bioimpedance of the object.    
   
   
       62 . The method of  claim 61 , further comprising calculating a cross-correlation signal, wherein the cross-correlation signal is a function of a time delay τ between the object excitation signal and the object response signal, wherein the cross-correlation signal is calculated by correlating the object excitation signal with the object response signal after delay of the object response signal by the time delay τ with respect to the object excitation signal.  
   
   
       63 . The method of  claim 61 , further comprising calculating a complex Fourier transform of the cross-correlation signal, to obtain complex values proportional to a complex bioimpedance.  
   
   
       64 . An apparatus for digital demodulation and further processing of signals obtained to measure electrical bioimpedance in an object, the apparatus comprising: 
 a voltage controlled current source for generating an excitation signal of known frequency content;    a current monitor coupled to the voltage controlled current source;    a first electrode coupled to the current monitor;    a second electrode coupled to the current monitor, wherein the first and second electrodes are configured for applying the excitation signal to the object;    a differential amplifier;    a third electrode coupled to the differential amplifier;    a fourth electrode coupled to the differential amplifier, wherein the third and fourth electrodes are configured for sensing a response signal across the object due to application of the excitation signal;    a first analog to digital converter coupled to the differential amplifier, wherein the differential amplifier and the first analog to digital converter are configured for acquiring, sampling and digitizing the response signal, to obtain a digitized response signal representing the response signal with respect to frequency content, amplitude and phase;    a first buffer coupled to the first analog to digital converter for temporarily storing the digitized response signal;    a multiplier/accumulator coupled to the first buffer, for correlating, for each frequency f AC  of the excitation signal applied, digitized samples of the digitized response signal, with corresponding discrete values representing the excitation signal; and    a processing unit coupled to the multiplier/accumulator, wherein the processing unit is configured to calculate, for each frequency f AC  of the excitation signal applied, complex values for the bioimpedance, from output values received from the multiplier/accumulator, and is further configured to provide, over time, a set of digital bioimpedance waveforms.    
   
   
       65 . The apparatus of  claim 64 , further comprising: 
 a filter coupled to the processing unit;    a monitor coupled to the filter;    a second analog to digital converter coupled to the current monitor, wherein the current monitor and the second analog to digital converter are configured for acquiring, sampling and digitizing the excitation signal to obtain a digitized excitation signal representing the excitation signal with respect to frequency content, amplitude and phase; and    a second buffer coupled to the second analog to digital converter, for temporarily storing the digitized excitation signal.    
   
   
       66 . The apparatus of  claim 65 , wherein the second analog to digital converter is adapted to sample at a rate that is in a range from about 4 to about 20 times the highest frequency of the excitation signal.  
   
   
       67 . The apparatus of  claim 65 , further comprising: 
 a timing control;    a plurality of multiplier/accumulators coupled to the timing control and the first buffer, for correlating, for each frequency f AC  of the excitation signal applied, digitized samples of the digitized response signal, with corresponding discrete values representing the excitation signal.    
   
   
       68 . The apparatus of  claim 67 , wherein each multiplier/accumulator outputs a respective digital waveform, and wherein each multiplier/accumulator is coupled to a first filter set adapted to separate the base bioimpedance Z 0 (f AC ) from each digital waveform.  
   
   
       69 . The apparatus of  claim 67 , wherein each multiplier/accumulator outputs a respective digital waveform, and wherein each multiplier/accumulator is coupled to a second filter set adapted to separate the changes in the bioimpedance ΔZ(f AC ,t) from each digital waveform, and wherein each multiplier/accumulator is coupled to a differentiator for obtaining a rate of change of the changes in bioimpedance.  
   
   
       70 . The apparatus of  claim 67 , wherein each multiplier/accumulator outputs a respective digital waveform, and wherein each multiplier/accumulator is coupled to a differentiator for obtaining a rate of change in the bioimpedance waveforms dZ(f AC ,t)/dt of the object.  
   
   
       71 . The apparatus of  claim 67 , wherein: 
 the plurality of multiplier/accumulators comprises first, second, third, and fourth multiplier/accumulators;    the first multiplier/accumulator in the plurality of multiplier/accumulators is configured to determine, for a first frequency f AC  of the excitation signal applied, an in-phase portion Re(Z(f AC ,t)) of the bioimpedance Z(f AC ,t);    the second multiplier/accumulator in the plurality of multiplier/accumulators is configured to determine, for the first frequency f AC  of the excitation signal applied, a quadrature portion Im(Z(f AC ,t)) of the bioimpedance z(f AC ,t);    the third multiplier/accumulator in the plurality of multiplier/accumulators is configured to determine, for a second frequency f AC  of the excitation signal applied, an in-phase portion Re(Z(f AC ,t)) of the bioimpedance z(f AC ,t); and    the fourth multiplier/accumulator in the plurality of multiplier/accumulators is configured to determine, for the second frequency f AC  of the excitation signal applied, a quadrature portion Im(Z(f AC ,t)) of the bioimpedance Z(f AC ,t).    
   
   
       72 . The apparatus of  claim 67 , wherein the discrete values representing the excitation signal are discrete values of a sinusoidal reference signal for the excitation signal.  
   
   
       73 . The apparatus of  claim 67 , further comprising a differentiator coupled to the processing unit.  
   
   
       74 . The apparatus of  claim 64 , wherein the voltage controlled current source and the current monitor are configured to generate a sinusoidal excitation signal of a known single frequency f AC .  
   
   
       75 . The apparatus of  claim 64 , further comprising: 
 a first switch coupled to the current monitor and the first electrode;    a second switch coupled to the current monitor and the second electrode;    a third switch coupled to the differential amplifier and the third electrode;    a fourth switch coupled to the differential amplifier and the fourth electrode; and    a calibration impedance coupled to the first switch, the second switch, the third switch, and the fourth switch.    
   
   
       76 . The apparatus of  claim 64 , further comprising: 
 a sine table; and    a digital to analog converter coupled to the sine table and the voltage controlled current source, for generating the excitation signal.    
   
   
       77 . The apparatus of  claim 64 , wherein the voltage controlled current source and the current monitor are configured to generate a sinusoidal excitation signal in a range of frequencies from about 1 kHz to about 1 MHz.  
   
   
       78 . The apparatus of  claim 64 , wherein the voltage controlled current source and the current monitor are configured to generate an excitation alternating current (AC) having an amplitude in a range from about 0.01 mA to about 5 mA.  
   
   
       79 . An apparatus for digital demodulation and further processing of signals obtained to measure electrical bioadmittance in an object, the apparatus comprising: 
 a voltage controlled current source for generating an excitation signal of known frequency content;    a current monitor coupled to the voltage controlled current source;    a first electrode coupled to the current monitor;    a second electrode coupled to the current monitor, the first and second electrodes for applying the excitation signal to the object;    a differential amplifier;    a third electrode coupled to the differential amplifier;    a fourth electrode coupled to the differential amplifier, the third and fourth electrodes for sensing a response signal across the object due to the application of the excitation signal;    a first analog to digital converter coupled to the differential amplifier, the differential amplifier and the first analog to digital converter for acquiring, sampling and digitizing the response signal to obtain a digitized response signal representing the response signal with respect to frequency content, amplitude and phase;    a first buffer coupled to the first analog to digital converter for temporarily storing the digitized response signal;    a multiplier/accumulator coupled to the first buffer, for correlating, for each frequency f AC  of the excitation signal applied, digitized samples of the digitized response signal, with corresponding discrete values representing the excitation signal;    a processing unit coupled to the multiplier/accumulator, for calculating, for each frequency f AC  of the excitation signal applied, complex values for the bioadmittance, from output values received from the multiplier/accumulator, and for providing, over time, a set of digital bioadmittance waveforms;    a filter coupled to the processing unit; and    a monitor coupled to the filter.    
   
   
       80 . An apparatus for digital demodulation and further processing of signals obtained to measure electrical bioimpedance in an object, the apparatus comprising: 
 signal generating means for generating an excitation signal of known frequency content;    a first pair of electrodes for applying the excitation signal to the object;    a second pair of electrodes for sensing the response signal across the object due to the application of the excitation signal;    first measuring means for acquiring, sampling and digitizing the response signal to obtain a digitized response signal representing the response signal with respect to frequency content, amplitude and phase;    memory means for temporarily storing the digitized response signal; and    digital demodulation means for correlating, for each frequency f AC  of the excitation signal applied, digitized samples of the response signal with corresponding discrete values of a sinusoidal reference signal to the excitation signal.    
   
   
       81 . The apparatus of  claim 80 , further comprising: 
 processing means for calculating for each frequency f AC  of the excitation signal applied, complex values for the bioimpedance Z(f AC ) from output values of the digital demodulation means, and for providing, over time, a set of digital bioimpedance waveforms Z(f AC ,t); and    recording means for recording a temporal course of the base bioimpedance.    
   
   
       82 . The apparatus of  claim 81 , further comprising: 
 means for, for each frequency f AC  of the excitation signal applied, separating the base bioimpedance Z 0 (f AC ) from the corresponding bioimpedance waveform Z(f AC ,t); and    means for, for each frequency f AC  of the excitation signal applied, separating the changes of bioimpedance ΔZ(f AC ,t) from the corresponding bioimpedance waveform Z(f AC ,t).    
   
   
       83 . The apparatus of  claim 82 , further comprising: 
 means for determining a rate of change of the bioimpedance dZ(f AC ,t)/dt, for each frequency f AC  of the excitation signal applied; and    means for recording a temporal course of the base bioimpedance Z 0 (f AC ) and of the rate of change of the bioimpedance dZ(f AC ,t)/dt, for each frequency f AC  of the excitation signal applied.    
   
   
       84 . The apparatus of  claim 83 , further comprising: 
 means for determining a rate of change of the changes in bioimpedance d(ΔZ(f AC ,t))/dt, for each frequency f AC  of the excitation signal applied; and    means for recording a temporal course of the base bioimpedance Z 0 (f AC ) and of the changes of the bioimpedance ΔZ(f AC ,t), for each frequency f AC  of the excitation signal applied.    
   
   
       85 . The apparatus of  claim 80 , further comprising an addressable sine look-up table coupled to the signal generating means, and wherein the signal generating means is adapted to generate the excitation signal using discrete values of a sinusoidal waveform.  
   
   
       86 . The apparatus of  claim 80 , further comprising an addressable sine look-up table coupled to the signal generating means, and wherein the signal generating means is adapted to generate the excitation signal by superposition of a number of sinusoidal waveforms.  
   
   
       87 . An apparatus for digital demodulation and further processing of signals obtained to measure electrical bioimpedance in an object, the apparatus comprising: 
 signal generating means for generating an excitation signal of known frequency content;    a first pair of electrodes for applying the excitation signal to the object;    a second pair of electrodes for sensing a response signal across the object due to the application of the excitation signal;    first measuring means for acquiring, sampling and digitizing the response signal to obtain a digitized response signal representing the response signal with respect to frequency content, amplitude and phase;    second measuring means for acquiring, sampling and digitizing the excitation signal to obtain a digitized excitation signal representing the excitation signal with respect to frequency content, amplitude and phase;    memory means for temporarily storing the digitized response signal;    processing means for calculating for each frequency f AC  of the excitation signal applied, complex values for the bioimpedance Z(f AC ) from the output values of the digital demodulation means, and for providing, over time, a set of digital bioimpedance waveforms Z(f AC ,t));    differentiating means for obtaining the rate of change in the bioimpedance waveforms dZ(f AC ,t)/dt, and    recording means for recording the rate of change in bioimpedance waveforms.    
   
   
       88 . The apparatus of  claim 87 , further comprising: 
 a calibration impedance;    a plurality of means for switching, for selectively coupling the signal generating means and the first measuring means to the calibration impedance, to acquire Calibration Current Samples and Calibration Voltage Samples;    wherein the signal generating means generates an alternating current (AC);    first correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the calibration impedance, the Calibration Current Samples with discrete values of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the alternating current applied;    second correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the calibration impedance, the Calibration Current Samples with the discrete values of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the alternating current applied;    third correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the calibration impedance, the Calibration Voltage Samples with the discrete values of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the voltage measured; and    fourth correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the calibration impedance, the Calibration Voltage Samples with the discrete values of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the voltage measured.    
   
   
       89 . The apparatus of  claim 88 , further comprising: 
 first calculating means for determining an equivalent to a current magnitude from the values proportional to the in-phase portion and the quadrature portion of the current applied,    second calculating means for determining an equivalent to a voltage magnitude from the values proportional to the in-phase portion and the quadrature portion of the voltage measured;    third calculating means for determining a current phase from the values proportional to the in-phase portion and the quadrature portion of the current applied;    fourth calculating means for determining a voltage phase of the values proportional to the in-phase portion and the quadrature portion of the voltage measured;    fifth calculating means for determining a system phase as the difference between the voltage phase and the current phase; and    sixth calculating means for determining an equivalent for the magnitude of the calibration impedance from the ratio of the equivalent for the voltage magnitude and the equivalent of the current magnitude.    
   
   
       90 . The apparatus of  claim 89:   wherein the plurality of means for switching are further adapted for selectively coupling the signal generating means and the first measuring means to the object, to acquire Object Current Samples and Object Voltage Samples;    and wherein the apparatus further comprises:    fifth correlating means for correlating, for each frequency f AC  of an alternating current (AC) applied to the object, the Object Current Samples with the discrete values of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the alternating current applied;    sixth correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the object, the Object Current Samples with the discrete values of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the alternating current applied;    seventh correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the object, the Object Voltage Samples with the discrete values of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the voltage measured; and    eighth correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the object, the Object Voltage Samples with the discrete values of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the voltage measured.    
   
   
       91 . The apparatus of  claim 90 , further comprising: 
 seventh calculating means for determining an equivalent to a current magnitude from values proportional to the in-phase portion and the quadrature portion of the current applied;    eighth calculating means for determining an equivalent to a voltage magnitude from the values proportional to the in-phase portion and the quadrature portion of the voltage measured;    ninth calculating means for determining a current phase from the values proportional to the in-phase portion and the quadrature portion of the current applied;    tenth calculating means for determining a voltage phase from the values proportional to in-phase portion and quadrature portion of the voltage measured;    eleventh calculating means for determining an object phase as the difference between the voltage phase and the current phase, corrected for the system phase;    twelfth calculating means for determining an equivalent for a magnitude of the object bioimpedance from the ratio of the equivalent for the voltage magnitude and the equivalent of the current magnitude;    thirteenth calculating means for determining a magnitude of the object bioimpedance from the ratio of the a priori known magnitude of the calibration impedance and the equivalent for the calibration impedance magnitude, multiplied by the equivalent for the object bioimpedance magnitude;    fourteenth calculating means for determining an in-phase portion of the object bioimpedance; and    fifteenth calculating means for calculating a quadrature portion of the object bioimpedance, from the magnitude and phase of the object bioimpedance.    
   
   
       92 . An apparatus for digital demodulation and further processing of signals obtained to measure electrical bioimpedance in an object, the apparatus comprising: 
 signal generating means for generating an excitation signal of known frequency content;    a first pair of electrodes for applying the excitation signal to the object;    a second pair of electrodes for sensing a response signal across the object due to the application of the excitation signal;    first measuring means for acquiring, sampling and digitizing the response signal to obtain a digitized response signal representing the response signal with respect to frequency content, amplitude and phase;    second measuring means for digitizing a voltage that is directly proportional to, and in phase with, the excitation signal;    memory means for temporarily storing the digitized response signal;    digital demodulation means for correlating, for each frequency f AC  of the excitation signal applied, digitized samples of the digitized response signal with corresponding discrete values of a sinusoidal reference signal to the excitation signal;    processing means for calculating for each frequency f AC  of the excitation signal applied, complex values for the bioimpedance Z(f AC ) from the output values of the digital demodulation means, and for providing, over time, a set of digital bioimpedance waveforms Z(f AC ,t));    separating means adapted to separate changes in the bioimpedance ΔZ(f AC ,t) from the waveforms;    differentiating means for obtaining a rate of change of the changes in bioimpedance d(ΔZ(f AC ,t))/dt; and    recording means for recording a temporal course of the base bioimpedance and of the changes in bioimpedance.    
   
   
       93 . The apparatus of  claim 92 , further comprising: 
 a calibration impedance;    a plurality of means for switching, for selectively coupling the signal generating means and the first measuring means to the calibration impedance, to acquire Calibration Current Samples and Calibration Voltage Samples;    wherein the signal generating means generates an alternating current (AC);    first correlating means for correlating, for a frequency f AC  of the alternating current (AC) applied to the object, the Calibration Current Samples with the Calibration Voltage Samples in order to obtain a value proportional to an in-phase portion of the calibration impedance;    second correlating means for correlating, for the frequency f AC  of the alternating current (AC) applied to the object, the Calibration Current Samples with the Calibration Voltage Samples samples, which are shifted in time by −90 degrees, in order to obtain a value proportional to a quadrature portion of the calibration impedance;    first calculating means for calculating an equivalent to a magnitude of the calibration impedance from the in-phase portion and quadrature portion; and    second calculating means for calculating a phase of the calibration impedance from the in-phase portion and quadrature portion.    
   
   
       94 . The apparatus of  claim 93:   wherein the plurality of means for switching are further adapted for selectively coupling the signal generating means and the first measuring means to the object, to acquire Object Current Samples and Object Voltage Samples;    and wherein the apparatus further comprises:    third correlating means for correlating, for a frequency f AC  of the alternating current (AC) applied to the object, the Object Current Samples with the Object Voltage Samples in order to obtain a value proportional to an in-phase portion of the calibration impedance;    fourth correlating means for correlating, for the frequency f AC  of the alternating current (AC) applied to the object, the Object Current Samples with the Object Voltage Samples samples, which are shifted in time by −90 degrees, in order to obtain a value proportional to a quadrature portion of the calibration impedance;    third calculating means for calculating an equivalent to a magnitude of the object bioimpedance from the in-phase portion and quadrature portion;    fourth calculating means for calculating an uncorrected phase of the object bioimpedance from the in-phase portion and quadrature portion;    fifth calculating means for calculating a correct phase of the object bioimpedance from the uncorrected object bioimpedance and from the phase of the calibration impedance; and    sixth calculating means for calculating a magnitude of the object bioimpedance from the ratio of an a priori known magnitude of the calibration impedance and the determined equivalent for the calibration impedance magnitude, multiplied by the determined equivalent for the object bioimpedance magnitude.    
   
   
       95 . The apparatus of  claim 94 , further comprising: 
 seventh calculating means for calculating an in-phase portion; and    eighth calculating means for calculating a quadrature portion from the magnitude and phase of the object bioimpedance.    
   
   
       96 . An apparatus for digital demodulation and further processing of signals obtained to measure electrical bioimpedance in an object, the apparatus comprising: 
 signal generating means for generating an excitation signal of known frequency content;    a first pair of electrodes for applying the excitation signal to the object;    a second pair of electrodes for sensing a response signal across the object due to the application of the excitation signal;    first measuring means for acquiring, sampling and digitizing the response signal to obtain a digitized response signal representing the response signal with respect to frequency content, amplitude and phase;    memory means for temporarily storing the digitized response signal;    digital demodulation means for correlating, for each frequency f AC  of the excitation signal applied, digitized samples of the digitized response signal with corresponding discrete values of a sinusoidal reference signal to the excitation signal;    processing means for calculating for each frequency f AC  of the excitation signal applied, complex values for the bioimpedance Z(f AC ) from output values of the digital demodulation means, and for providing, over time, a set of digital bioimpedance waveforms Z(f AC ,t));    first separating means adapted to separate a base bioimpedance Z 0 (f AC ) from the waveforms; and    recording means for recording a temporal course of the base bioimpedance.    
   
   
       97 . The apparatus of  claim 96 , further comprising: 
 a calibration impedance;    a plurality of means for switching, for selectively coupling the signal generating means and the first measuring means to the calibration impedance, to acquire Calibration Voltage Samples;    wherein the signal generating means generates an alternating current (AC);    first correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the calibration impedance, the Calibration Voltage Samples with the discrete values of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the calibration impedance;    second correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the calibration impedance, the Calibration Voltage Samples with the discrete values of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the calibration impedance;    first calculating means for determining an equivalent to a magnitude of the calibration impedance; and    second calculating means for determining a phase of the calibration impedance.    
   
   
       98 . The apparatus of  claim 97:   wherein the plurality of means for switching are further adapted for selectively coupling the signal generating means and the first measuring means to the object, to acquire Object Voltage Samples;    and wherein the apparatus further comprises:    third correlating means for correlating, for each alternating current frequency f AC  applied to the object, the Object Voltage Samples with samples of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the object bioimpedance;    fourth correlating means for correlating, for each alternating current frequency f AC  applied to the object, the Object Voltage Samples with the samples of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the object bioimpedance;    third calculation means for determining an equivalent to a magnitude of the object bioimpedance;    fourth calculation means for determining the uncorrected phase of the object bioimpedance;    fifth calculating means for determining a correct phase of the object bioimpedance by subtracting the phase of the calibration impedance from the phase of the uncorrected object bioimpedance;    sixth calculating means for determining a magnitude of the object bioimpedance from the ratio of an a priori known magnitude of the calibration impedance and the determined equivalent for the calibration impedance magnitude, multiplied by the determined equivalent for the object bioimpedance magnitude;    seventh calculating means for determining an in-phase portion of the object bioimpedance; and    eighth calculating means for determining a quadrature portion of the object bioimpedance from the magnitude and phase of the object bioimpedance.    
   
   
       99 . The apparatus of  claim 98 , further comprising: 
 fitting means for fitting samples of the digitized current signals of the calibration impedance towards discrete values of an ideal sinusoidal waveform, and for providing, over time, Calibration Current Samples;    fitting means for fitting samples of the digitized voltage signals of the calibration impedance towards values of an ideal sinusoidal waveform, and for providing, over time, the Calibration Voltage Samples.    
   
   
       100 . The apparatus of  claim 96 , further comprising second measuring means for digitizing a voltage that is directly proportional to, and in phase with, the excitation signal.  
   
   
       101 . The apparatus of  claim 96 , further comprising: 
 a plurality of means for switching, for selectively coupling the signal generating means and the first measuring means to the object, to acquire Object Current Samples and Object Voltage Samples;    and wherein the signal generating means generates an alternating current (AC);    and further comprising:    first correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the object, the Object Current Samples with discrete values of an ideal sine waveform in order to obtain a value proportional to an in-phase portion of the alternating current;    second correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the object, the Object Current Samples with discrete values of an ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the alternating current;    third correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the object, the Object Voltage Samples with discrete values of another ideal sine waveform to obtain a value proportional to an in-phase portion of the voltage;    fourth correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied to the object, the Object Voltage Samples with discrete values of another ideal cosine waveform in order to obtain a value proportional to a quadrature portion of the voltage;    first calculation means for determining a current magnitude from the values proportional to in-phase portion and quadrature portion of the current applied;    second calculation means for determining a voltage magnitude from the values proportional to the in-phase portion and the quadrature portion of the voltage measured;    third calculation means for determining a current phase of the values proportional to the in-phase portion and the quadrature portion of the current applied;    fourth calculation means for determining a voltage phase from the values proportional to the in-phase portion and the quadrature portion of the voltage measured;    fifth calculation means for determining a phase of the object bioimpedance as the difference between the voltage phase and the current phase;    sixth calculation means for determining a magnitude of the object bioimpedance from the ratio of voltage magnitude and current magnitude;    seventh calculating means for determining an in-phase portion of the object bioimpedance; and    eighth calculating means for determining a quadrature portion of the object bioimpedance from the magnitude and phase of the object bioimpedance.    
   
   
       102 . The apparatus of  claim 96 , further comprising: 
 a plurality of means for switching, for selectively coupling the signal generating means and the first measuring means to the object, to acquire Object Current Samples and Object Voltage Samples;    and wherein the signal generating means generates an alternating current (AC);    and further comprising:    first correlating means for correlating the Object Current Samples with the Object Voltage Samples in order to obtain a value proportional to an in-phase portion of the object bioimpedance;    second correlating means for correlating the Object Current Samples with the Object Voltage Samples, which are shifted in time by −90 degrees, in order to obtain a value proportional to a quadrature portion of the object bioimpedance;    first calculating means for determining a magnitude of the object bioimpedance; and    second calculating means for determining a phase of the object bioimpedance, both from the in-phase portion and quadrature portion of the object bioimpedance.    
   
   
       103 . The apparatus of  claim 96 , further comprising: 
 a plurality of means for switching, for selectively coupling the signal generating means and the first measuring means to the object, to acquire Object Voltage Samples;    and wherein the signal generating means generates an alternating current (AC) having constant magnitude;    and further comprising:    sampling means for providing, for each frequency f AC  of the alternating current (AC) applied, discrete values of an ideal sine waveform which represent the current in magnitude and phase, wherein the discrete values of the ideal sine waveform are called Reference Current Samples (REF);    first correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied, the Object Voltage Samples with the Reference Current Samples (REF), to obtain a value proportional to an in-phase portion of the object bioimpedance;    second correlating means for correlating, for each frequency f AC  of the alternating current (AC) applied, the Object Voltage Samples with the Reference Current Samples (REF), to obtain a value proportional to a quadrature portion of the object bioimpedance;    first calculating means for determining a magnitude of the object bioimpedance; and    second calculating means for determining a phase of the object bioimpedance both from the in-phase portion and quadrature portion of the object bioimpedance.    
   
   
       104 . The apparatus of  claim 103 , further comprising: 
 fitting means for fitting the samples of the digitized voltage signals of the object towards values of an ideal sinusoidal waveform, and for providing, over time, the Object Voltage Samples.    
   
   
       105 . An apparatus for digital demodulation and further processing of signals obtained to measure electrical bioadmittance in an object, the apparatus comprising: 
 signal generating means for generating an excitation signal of known frequency content;    a first pair of electrodes for applying the excitation signal to the object;    a second pair of electrodes for sensing a response signal across the object due to application of the excitation signal;    first measuring means for acquiring, sampling and digitizing the response signal to obtain a digitized response signal representing the response signal with respect to frequency content, amplitude and phase;    memory means for temporarily storing the digitized response signal;    digital demodulation means for correlating, for each frequency f AC  of the excitation signal applied, digitized samples of the digitized response signal with corresponding discrete values of a sinusoidal reference signal to the excitation signal;    processing means for calculating for each frequency f AC  of the excitation signal applied, complex values for the bioadmittance Y(f AC ) from output values of the digital demodulation means, and for providing, over time, a set of digital bioadmittance waveforms Y(f AC ,t);    means for, for each frequency f AC  of the excitation signal applied, separating the base bioadmittance Y 0 (f AC ) from the bioadmittance waveform Y(f AC ,t);    means for, for each frequency f AC  of the excitation signal applied, separating the changes of bioadmittance ΔY(f AC ,t) from the bioadmittance waveform Y(f AC ,t); and    recording means for recording a temporal course of the base bioadmittance Y 0 (f AC ).