Study on a diode-bridge type capacitance detection circuit for differential capacitive sensor

Diode-bridge type capacitance detection circuit was developed in order to detect very small variation of differential capacitances in higher resolution. The circuit was designed for a comb structure tilt sensor, which was made of quartz crystal and had several movable electrodes between fixed electrodes. The circuit's output was almost proportional to the displacement of the movable electrode. Over 100kHz AC voltage was supplied to obtain output signal sufficiently. In order to prevent decrease of the sensitivity, the diode-bridge part was composed of commercial PIN diodes, which had very low junction capacitance (less than 1pF), and was mounted in small ceramic package with the tilt sensor. At present, the resolution of 0.0015 degree was obtained using the combination of the sensor and the detection circuit. It corresponded to the resolution of about less than 4.5aF.