LOW-COST CMOS INTERFACE FOR CAPACITIVE SENSORS AND ITS APPLICATION INA CAPACITIVE ANGULAR ENCODER

A low-cost CMOS integrated interface for capacitive sensors is present ed. The interface is composed of two separate chips: a capacitance-con trolled oscillator and a selector, which result in a structure that is able to measure several capacitances accurately and has a microcontro ller-compatible output. In this interface, even large parasitic capaci tances of up to 3 nF between the terminals of the capacitor to be meas ured and ground are allowed. Prototypes of the interface chips have be en fabricated in a 0.7-mu m CMOS process. The frequency of the oscilla tor amounts to 90 kHz. For the capacitance measurement, the interface has a resolution of 11.3 ppm and a nonlinearity of 300 ppm over a meas urement range of 2 pF In this paper, also the application of the inter face in an accurate capacitive angular encoder is discussed.