An integrated interface circuit with a capacitance-to-voltage converter as front-end for grounded capacitive sensors

This paper presents the analysis and design of an integrated interface for grounded capacitive sensors. To reduce the effects of parasitic cable capacitances, a feedforward technique has been applied. In combination with the use of a special front-end amplifier this yields high immunity for a parasitic cable capacitance. The major nonidealities of the interface circuit have been analyzed. The complete interface has been designed and implemented as an integrated circuit, using standard 0.7 mu m CMOS technology. Experimental results, which are in good agreement with theoretical analysis and simulation, show that for sensor capacitance down to 10 pF, shielded connection cables up to 30 m can be handled with an absolute error of less than 0.3 pF. The measured nonlinearity of the interface amounts to about 3 x 10(-4) for 30 m of the cable. For 40 ms measurement time, the resolution amounts to about 16 bits.