Capacitive-Sensor Interface With High Accuracy and Stability

This paper presents a capacitive-sensor interface with high accuracy and excellent stability. The range of the measured capacitance is up to 1 pF. The implemented circuit tolerates cable capacitances of up to 1 nF. Depending on the operation mode, the conversion time varies between 39 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. A special multislope analog-to-digital conversion (ADC) is used, which, to increase the resolution, transfers itself at the beginning and at the end of the conversion process into an oversampling conversion. Based on the band-pass filtering function of the implemented ADC technique and the low-noise input stage used, a dynamic range of better than 14 b has been achieved for a cable capacitance that is less than 400 pF.