A novel interface circuit for grounded capacitive sensors with feedforward-based active shielding

This paper proposes and analyses a novel interface circuit for capacitive sensors in which one of the electrodes is grounded. The novel design makes a charge-balanced relaxation oscillator ( applied so far to floating capacitive sensors) suited for the measurement of grounded capacitive sensors and applies advanced measurement techniques, such as auto-calibration and chopping. Furthermore, these techniques are combined with feedforward-based active shielding instead of the usual feedback-based one, thus avoiding instability problems. A prototype of the novel interface circuit has been implemented with discrete components and tested for sensor capacitances between 27 pF and 330 pF and for different lengths of the interconnecting cable. The nonlinearity error amounts to less than 0.1% FSS for a 10 m cable.