Measurement of small capacitance variations

A technique for measuring low-frequency and low-level capacitance variations is proposed. It is based on a lock-in detection circuit with a feedback loop, containing an integrator and a modulator for zeroing the capacitance mean value. This approach provides a good signal-to-noise ratio and high sensitivity. Capacitance variations can be on the order of 100 p.p.m. of the mean value, and the frequency of the variations can be as low as 0.1 Hz. Stray capacitances and the drift due to the environmental conditions are automatically compensated. The measurement technique, experimental apparatus, and initial results are described. Small variations, about 10 fF, have been measured in the presence of 10-pF mean value.<>