A Fast Interface for Low-value Capacitive Sensors with Improved Accuracy

This paper presents a fast Interface for multiple capacitive sensors with Improved accuracy and extended functionality. The range of the measured capacitance Is up to 1 pF. The Implemented circuit tolerates cable capacitance of up to 1 nF. Depending on the operation mode, the conversion time varies between 39 mus and 100 mus. The measurement principle permits a mode with compensation of small sensor leakage currents. The reference units are frequency, resistance and resistance ratios, which provide high initial accuracy and a long- term stability. The measured temperature coefficient of the capacitance converter is about 2 ppm/K. The dynamic range is better than 14 bits for cable capacitance less than 400 pF. A special multi-slope A-to-D conversion is used, which, to increase the resolution, at the beginning and at the end of the conversion process transfers itself into an over-sampling conversion.