AN-C2V: An Auto-Nulling Bridge-Based Signal Conditioning Circuit for Leaky Capacitive Sensors

This paper presents a fully analog auto-nulling bridge based capacitance-to-voltage (AN-C2V) converter circuit for measuring the capacitive component of the leaky capacitive sensors. The circuit utilizes an active bridge based architecture in which the quadrature components of the output are taken. An integrator-controller is used for auto-nulling. A unity gain, frequency-independent quadrature phase shift amplifier is designed for an accurate generation of 90° phase-shifted signal. Experiments conducted on the developed prototype demonstrate that the proposed AN-C2V system is able to estimate the sensor capacitor ranging from 47.2pF to 668.3pF with a maximum error of about ± 0.18% with respect to the commercial LCR meter. The effectiveness of the circuit is tested with the commercial leaky capacitive humidity sensor. The effect of leakage resistor, parasitic capacitor, frequency of excitation signal, and supply voltage is negligible on the final output, which results in a very robust and accurate solution for leaky capacitive sensors.