Interface circuit for measuring small capacitance changes in sensor networks

This study describes a novel circuit configuration, capable of handling fast transients and small capacitance changes by the evaluation of the signals of a capacitive sensor network. The circuit is based on a conventional bridge measurement method being complemented with a control loop and high-order filters in order to cancel the effect of the environmental interference, the in-circuit noise, the propagation delay of the measuring signal and the parasitic elements. Through the first circuit tests, applying to the proposed measurement configuration, it was possible to detect capacitance changes in the order of 100 ppm of the nominal capacitance value with a sampling frequency of 2 kHz. The state of the art measurement configurations compare the rectified measurement signal of the capacitive sensor with that of a reference capacitance. The difference signal, already having a considerable noise content because of the diode-rectification is then further amplified. The measurement configuration proposed in the study is however amplifying the difference of the unrectified signals. Owing to the rectification being applied only after the gain stage, the noise of the rectifier unit is considerably smaller compared with other solutions. This method is having further potentials especially by means of measurement speed and accuracy.