A novel AC current source for capacitance-based displacement measurements

A 100 kHz bandwidth AC current source suitable to be used with capacitive sensors in displacement measurement applications has been designed and constructed. The high output impedance of the source is obtained by sensing the load voltage with a buffer stage having very low input capacitance. The sensing-buffer power supply, its shield, and the input shielding connections are driven at the buffer input potential in such a way that the effect of all input parasitic admittances, including the buffer common-mode impedance, are almost suppressed. The load impedance variations, which are directly proportional to displacements, are monitored by the output voltage of the sensing buffer. Measurements made at different frequencies with calibrated capacitance loads ranging from (1 to 20) pF have shown that the load-impedance values are proportional to the output voltage of the sensing buffer with a linearity better than 4/spl times/10/sup -4/ and resolution of about 3/spl times/10/sup -5/ over a 1 kHz bandwidth of load variation signal. The features of the current source and its performance in a displacement measurement application are described.