Novel switched-capacitor dual slope capacitance to digital converter for differential capacitive sensors

A novel, simple and high accuracy dual slope direct digital converter suitable for differential capacitive sensors is presented in this work. The proposed technique provides a linear digital output proportional to the parameter being sensed by differential capacitive sensors irrespective of the fact that the sensor could possess either a linear or an inverse characteristic. In the proposed technique, the output is dependent only on, apart from the sensitivity of the sensor, a single dc reference voltage. Thus by employing a precision dc reference voltage, high accuracy is easily achieved. The proposed direct capacitance to digital converter is based on the popular dual-slope analog to digital converter structure and hence possesses all the advantages of the dual-slope technique, namely, high resolution, accuracy and immunity to noise and component parameter variations. A prototype built and tested for a typical differential capacitive sensor with a nominal value of 250 pF gave a worst-case error less than 0.05 %.