Xj. Li et al., LOW-COST CMOS INTERFACE FOR CAPACITIVE SENSORS AND ITS APPLICATION INA CAPACITIVE ANGULAR ENCODER, Analog integrated circuits and signal processing, 14(3), 1997, pp. 223-233
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.