Abstract
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.
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