An interface circuit for grounded and leaky capacitive sensors

The paper presents an interface circuit for capacitive sensors that are shunted by a parasitic leakage conductance and which are grounded at one of their electrode terminals. The electronic circuit is designed based-on the use of a relaxation oscillator. In the designed interface, the effect of shunting conductance is reduced by using the charge/discharge-balanced method. The parasitic effects of the guarding electrode and stray capacitances are eliminated by using so-called feedforward-based active-shielding technique. The use of auto-calibration and chopping eliminates all multiplicative and additive errors of the interface. A prototype has been built for sensor capacitances in the range of the 0 pF ~ 330 pF with shunting conductance up to Gs = 10 μS and a measurement time of 7.5 ms. The experimental results show a maximum error of 2.3 pF (0.7%FS) and a standard deviation due to noise of 12.7 fF (0.004%FS).