Control circuit for use with a four terminal sensor, and measurement system including such a control circuit

Abstract

A control circuit for use with a four terminal sensor, the sensor having first and second drive terminals and first and second measurement terminals, the control circuit arranged to drive at least one of the first and second drive terminals with an excitation signal, to sense a voltage difference between the first and second measurement terminals, and control the excitation signal such that the voltage difference between the first and second measurement terminals is within a target range of voltages, and wherein the control circuit includes N poles in its transfer characteristic and N−1 zeros in its transfer characteristic such that when a loop gain falls to unity the phase shift around a closed loop is not substantially 2π radians or a multiple thereof, where N is greater than 1.

1 . A method of operating an instrument loop comprising a multi terminal sensor and an excitation circuit, wherein the multi-terminal sensor has at least one drive terminal which is distinct from a first sense terminal, and wherein the multi-terminal sensor has further terminal, and wherein the excitation circuit is arranged to measure a voltage difference between the first sense terminal and the further terminal, and to use this voltage difference to control an excitation signal applied to the at least one drive terminal so as to maintain the voltage difference between the first sense terminal and the further terminal to a target value or to within a target range, and wherein the excitation circuit is arranged to have at least one zero in its transfer characteristic such that it satisfies the Barkhausen stability criterion. 
     
     
         2 . The method of  claim 1 , wherein the excitation circuit has N poles and N−1 zeros in its transfer characteristic. 
     
     
         3 . The method of  claim 2 , wherein the poles are provided as integrators. 
     
     
         4 . The method of  claim 1 , further comprising providing the excitation signal with or as an AC component at a plurality of frequencies, and measuring a complex impedance of the multi-terminal sensor at a plurality of frequencies. 
     
     
         5 . The method of  claim 4 , further comprising using the measurement of complex impedance to compensate a parameter of the multi-terminal sensor. 
     
     
         6 . The method of  claim 5 , wherein the sensor includes a chemical or biological sensing cell, and the parameter that is compensated is temperature within the sensing cell. 
     
     
         7 . The method of  claim 1 , wherein the N−1 zeros occur at frequencies at which the control loop has a gain of more than unity. 
     
     
         8 . The method of  claim 1 , wherein the N−1 zeros occur at frequencies at which the control loop has a gain of more than 0.5. 
     
     
         9 . The method of  claim 1 , wherein the N poles occur at frequencies at which the control loop has a gain of more than unity. 
     
     
         10 . The method of  claim 1 , wherein the sensor is a four terminal glucose sensor.