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 μm 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 × 10−4 for 30 m of the cable. For 40 ms measurement time, the resolution amounts to about 16 bits.