A fast charge-meter for interfacing capacitive sensors

This paper presents a fast charge-meter as an electronic interface for multiple capacitive sensors with maximum value lpF. A cable capacitance of up to InF can be tolerated. Two different modes of measurement are available - a normal mode and a fast mode. The conversion time is 39 mus and 64 mus for the fast and for the normal mode, respectively. The resolution in normal mode, with cable capacitance less than 400 pF, is better than 14 bits. A special multi-slope A-to-D conversion is used, which, to increase the resolution, at the beginning and at the end of the conversion process transfers itself into an over-sampling conversion. The interface is realized on a PCB with a single euro-board format. Six sensors can be interfaced in programmable sequence. The power consumption is less than 1 W.