Current measurement system

Abstract

A measurement system for measuring an input electrical current (Ics) from a current source (CS) and generating a current measurement signal, comprising a current measuring circuit ( 70 ) having a first input terminal ( 72 ) connected to the current source and an output terminal ( 74 ) for providing the current measurement signal. The current measuring circuit further comprises one or more power supply terminals ( 75, 76 ) arranged to receive one or more voltages from a power supply ( 77 a, 77 b) for powering the current measuring circuit. The current measuring circuit also comprises a first voltage source (VD) coupled to the one or more power supply terminals, the first voltage source providing a disturbance voltage to the one or more power supply terminals, the disturbance voltage representing a voltage at the first input terminal.

The invention claimed is: 
     
       1. A measurement system for measuring an input electrical current from a current source and generating a current measurement signal, comprising a current measuring circuit having a first input terminal connected to the current source for receiving the input electrical current from the current source and an output terminal for providing the current measurement signal,
 wherein the current measuring circuit further comprises one or more power supply terminals arranged to receive one or more voltages from a power supply for powering the current measuring circuit, and 
 wherein the current measuring circuit further comprises a first voltage source coupled to the one or more power supply terminals, the first voltage source providing is configured to provide a disturbance voltage to the one or more power supply terminals, the disturbance voltage representing a voltage at the first input terminal, 
 wherein the first input terminal, the output terminal and the one or more power supply terminals are distinct, 
 wherein the current source, the power supply and the first voltage source are distinct, 
 and wherein the measurement circuit is configured to drive the one or more power supply voltages with a voltage essentially the same as a voltage present at the first input terminal by feeding the disturbance voltage to the one or more power supply terminals. 
 
     
     
       2. The measurement system of  claim 1 , further comprising a difference circuit arranged to subtract a voltage generated by the first voltage source from a signal at the output terminal of the current measuring circuit to generate the current measurement signal. 
     
     
       3. The measurement system of  claim 1 , wherein the first voltage source is connected to the first input terminal of the current measuring circuit for driving a load to form the current source. 
     
     
       4. The measurement system of  claim 3 , wherein the load comprises a capacitive sensor for generating a current which varies in dependence on distance between the capacitive sensor and a target. 
     
     
       5. The measurement system of  claim 3 , wherein the load is connected to the first input terminal of the current measuring circuit by a cable comprising a sensor wire and a shield conductor, wherein the sensor wire is connected in series between the load and the first input terminal and the shield conductor is connected to the first voltage source. 
     
     
       6. The measurement system of  claim 1 , wherein an output terminal of the first voltage source is coupled via one or more capacitors to the one or more power supply terminals of the current measuring circuit. 
     
     
       7. The measurement system of  claim 1 , wherein the current measuring circuit comprises a current-to-voltage converter. 
     
     
       8. The measurement system of  claim 1 , wherein the current measuring circuit comprises an operational amplifier, a negative input terminal of the operational amplifier serving as the first input terminal of the current measuring circuit and an output terminal of the operational amplifier serving as the output terminal of the current measuring circuit, the operational amplifier further comprising a positive input terminal and one or more power supply terminals, wherein the positive input terminal of the operational amplifier is electrically connected to the one or more power supply terminals of the operational amplifier. 
     
     
       9. The measurement system of  claim 8 , wherein the positive input terminal of the operational amplifier is electrically connected to the one or more power supply terminals of the operational amplifier via one or more capacitors. 
     
     
       10. The measurement system of  claim 1 , wherein the first voltage source generates a voltage with a triangular waveform. 
     
     
       11. The measurement system of  claim 1 , wherein the current source generates a current with a substantially square waveform. 
     
     
       12. A method for measuring an input electrical current from a current source and generating a current measurement signal, comprising:
 providing the input electrical current from the current source to a first input terminal of a current measuring circuit, the current measuring circuit having one or more power supply terminals arranged to receive one or more power supply voltages from a power supply for powering the current measuring circuit, and 
 providing a disturbance voltage from a voltage source to the one or more power supply terminals, the disturbance voltage representing a voltage at the first input terminal, hence driving the one or more power supply voltages with a voltage essentially the same as a voltage present at the first input terminal, and 
 generating an output signal at an output terminal of the current measuring circuit representing the input electrical current at the first input terminal of the current measuring circuit, 
 wherein the first input terminal, the output terminal and the one or more power supply terminals are distinct, and 
 wherein the current source, the power supply and the first voltage source are distinct. 
 
     
     
       13. The method of  claim 12 , further comprising subtracting the disturbance voltage from the output signal at the output terminal of the current measuring circuit to generate the current measurement signal. 
     
     
       14. The method of  claim 12 , further comprising driving a load with a voltage to generate the input electrical current at the first input terminal of the current measuring circuit. 
     
     
       15. The method of  claim 14 , wherein the load comprises a capacitive sensor for generating a current which varies in dependence on distance between the capacitive sensor and a target. 
     
     
       16. The method of  claim 14 , further comprising connecting the load to the first input terminal of the current measuring circuit by a cable comprising a sensor wire and a shield conductor, wherein the sensor wire is connected in series between the load and the first input terminal and the shield conductor is energized with substantially the same voltage used to drive the load. 
     
     
       17. The method of  claim 12 , wherein the disturbance voltage is provided to the one or more power supply terminals via one or more capacitors. 
     
     
       18. The method of  claim 12 , wherein the disturbance voltage is isolated from the power supply voltages by one or more inductors. 
     
     
       19. The measurement system of  claim 1 , wherein the current measurement circuit is coupled to a cable comprising a sensor wire and a shield conductor, wherein the sensor wire is coupled to the first input terminal and the shield conductor is coupled to the first voltage source. 
     
     
       20. The measurement system of  claim 19 , wherein the cable is connected to a capacitive sensor at an end of the cable remote from the measurement circuit, the capacitive sensor comprising a sensor electrode and a guard electrode, wherein the sensor electrode is electrically connected to the sensor wire and the guard electrode is electrically connected to the shield conductor. 
     
     
       21. The measurement system of  claim 1 , wherein the power supply for powering the current measurement circuit is arranged to supply a DC voltage to the one or more power supply terminals. 
     
     
       22. The measurement system of  claim 1 , wherein the disturbance voltage is an AC voltage. 
     
     
       23. The measurement system of  claim 1 , wherein one or more inductors are provided between the power supply and the first voltage source for isolating the disturbance voltage from the one or more voltages from the power supply. 
     
     
       24. The measurement system of  claim 1 , wherein the first voltage source is further coupled to a second input terminal of the current measurement circuit. 
     
     
       25. The measurement system of  claim 1 , wherein the current measurement system comprises an operational amplifier and wherein the first input terminal, the output terminal and the one or more power supply terminals are terminals of the operational amplifier. 
     
     
       26. An inspection tool comprising:
 a measurement system configured to measure an input electrical signal from a capacitive sensor and to generate a current signal, the measurement system comprising:
 a circuit comprising:
 a first input terminal connected to the capacitive sensor and configured to receive the input electrical current from the capacitive sensor; 
 one or more power supply terminals arranged to receive one or more power supply voltages from a power supply configured to power the circuit; 
 a first voltage source coupled to the one or more power supply terminals and configured to provide a disturbance voltage to the one or more power supply terminals, the disturbance voltage representative of a voltage at the first input terminal; and 
 an output terminal configured to provide the current signal, 
 
 wherein the circuit is configured to drive the one or more power supply voltages with a voltage essentially the same as a voltage present at the first input terminal by directing the disturbance voltage to the one or more power supply terminals, 
 wherein the first input terminal, the output terminal, and the one or more power supply terminals are distinct, and 
 wherein the capacitive sensor, the power supply, and the first voltage source are distinct.  
   
     
     
       27. The inspection tool of claim 26, wherein the first voltage source is coupled to the first input terminal and is configured to drive the capacitive sensor.  
     
     
       28. The inspection tool of claim 26, further comprising a projection system having a lens.  
     
     
       29. The inspection tool of claim 28, further comprising a stage configured to support a substrate.  
     
     
       30. The inspection tool of claim 29, wherein the capacitive sensor is configured to generate the input electrical current dependent on the distance corresponding to the projection system and the stage and/or substrate.  
     
     
       31. The inspection tool of claim 29, wherein the disturbance voltage is representative of a change in distance corresponding to the projection system and an opposing surface of the substrate and/or the stage.  
     
     
       32. A capacitive measurement system configured to measure an input electrical signal from a capacitive sensor and to generate a current signal, the capacitive measurement system comprising:
 a first input terminal connected to the capacitive sensor and configured to receive the input electrical current from the capacitive sensor;   one or more power supply terminals arranged to receive one or more power supply voltages from a power supply;   a first voltage source coupled to the one or more power supply terminals and configured to provide a disturbance voltage to the one or more power supply terminals, the disturbance voltage representing a voltage at the first input terminal; and   an output terminal configured to provide the current signal,   wherein the one or more power supply voltages are driven with a voltage essentially the same as a voltage present at the first input terminal by directing the disturbance voltage to the one or more power supply terminals,   wherein the first input terminal, the output terminal, and the one or more power supply terminals are distinct, and   wherein the capacitive sensor, the power supply, and the first voltage source are distinct.