A Digital Interfacing Circuit Based on Sliding DFT Bins for Capacitive Sensors

A digital interfacing circuit (DIC) based on sliding discrete Fourier transform (SDFT) bins has been proposed for lossy capacitive sensors. The proposed interfacing circuit consisting of an analog sensing circuit and an analog-to-digital converter (ADC)-field-programmable gate array (FPGA)-based DIC. The sliding DFT bin-1 is employed as a quadrature signal generator to obtain in-phase and quadrature signals of the input supply voltage provided to the analog sensing circuit. The output sinusoidal signal of analog sensing circuit is correlated with the in-phase and quadrature signals separately and then averaged with SDFT bin-0 for computing sensing capacitance and resistance values. The proposed method is implemented in FPGA and tested for capacitance measurement range of 11 pF-5.27 nF with the maximum relative percentage error of 1.36%. The nonlinearity error is < 0.37% FS and FSS. Moreover, the suitability of the proposed interface for humidity measurement has been explored and validated experimentally with saturated salt solutions. Furthermore, the interdigitated capacitance is fabricated and tested with the proposed digital interface for the capacitance range of 30-122 pF, which finds an application in capacitive touchpad.