Fully analog automatic stray compensation for bridge-based differential capacitive sensor interfaces

In this paper, a novel automatic fully-analog feedback circuit able to dynamically compensate stray capacitances in bridge-based differential capacitive sensor interfaces, due to both the sensor itself and the read-out circuitry, is presented. It relies on the detection of the difference between currents flowing across the two sensor capacitors, so compensating possible imbalances through a suitably driven Voltage Controlled Negative Impedance Converter (VCNIC). The effectiveness of the proposed solution is proven by the reported case study, where the error on the output voltage is reduced from 23% (uncompensated) down to less than 1%. These features can extend the application field of a differential capacitive sensor interface showing very low variations, also in sub-pF range, without loss of accuracy.