Precision impedance variation measurement circuit

摘要

A condition sensing capacitor (10) is placed in an appropriate position such that its capacitance Cx varies in accordance with a condition to be sensed. A reference capacitor (12) and a feedback capacitor (14) are positioned in locations in which their capacitances Co and Cf are unaffected by the sensed condition. During a first time period, the sensing capacitor is connected with a source (22) of a first calibration voltage source Vg, the reference capacitor is connected with a source (26) of a second calibration voltage Vo and the feedback capacitor is connected with the output (34). During a second time period, the charge stored on the sensing capacitor is reduced by a current mirror (60) in proportion to the charge stored on the reference capacitor. The charge on the feedback capacitor is reduced by a current mirror (62) in accordance with charge stored in stray capacitance. During a third time period, the charge stored on the sensing and feedback capacitors is substractively combined (30) to produce a difference current which is applied across a current-to-voltage converter (32). The converter transforms the difference current into an output voltage. In this manner, the output voltage Vout varies with the difference between the product of the first calibration voltage and the condition sensing capacitance and the product of the second calibration voltage and the reference capacitance, i.e. Vout=(VgxCx-VoxCo)/Cf. The sensitivity S varies with the ratio of the first calibration voltage to the feedback capacitance, i.e. S=Vg/Cf. In this manner, the sensitivity can be adjusted by adjusting the first calibration voltage Vg and the output voltage can be nulled by adjusting the second calibration voltage Vo.