A novel modem model insensitive to the effect of the modulated carrier and the demodulated-signal phase adapted for capacitive sensors

The differential bridge-type capacitive sensors are widely applied in many precise instrumental measurement fields, where the modem is a key technology to realize the detection for a tiny differential capacitance. A universal structure based on a modulated sine carrier and a switched analog demodulation is adopted, so two factors including the carrier’s stability and the slight phase difference with its fluctuation between demodulated signals will inevitably limit the modem performance, and further deteriorate the capacitance sensitivity. In this paper, a novel modem based on a modulated step sine carrier is proposed, and the theoretical analysis indicates that the novel structure can significantly reduce the effects of aforesaid factors. Finally, a joint evaluation circuit is built to preliminarily measure the modem noise, and experimental results demonstrate a relative voltage noise power spectral density of 7 ppm/Hz1/2 at 1 mHz for the novel modem, two times better than that with a sine carrier. •The proposed modem based on a step sine carrier and a switched analog demodulation.•The detailed mechanism analysis for two models based on two different carriers.•A better performance for the model based on the step sine carrier in the analysis.•Relative noise PSD of about 7 ppm/Hz1/2 at 1 mHz for the model with a step sine carrier.