Switched - Capacitor Sigma - Delta Capacitance to Digital Converter Suitable for Differential Capacitive Sensors

A high accuracy sigma-delta capacitance to digital converter (CDC) that accepts differential capacitive sensors possessing either linear or inverse characteristic and provides a linear digital output is presented here. The sensor capacitances become an integral part of the delta modulator in a sigma-delta digital converter, with the switching sequenced to eliminate the effects of stray capacitances. High accuracy is obtained since the digital output is dictated only by a single dc reference voltage and the transformation constant of the sensor. A prototype built and tested exhibited a worst case error less than 0.15%, establishing the feasibility of the proposed sigma-delta CDC.