A Use Case of Sliding DFT in Capacitance and Resistance Sensors Interface

This article proposes a use case of sliding discrete Fourier transform (SDFT) in the capacitive and resistive sensors interface. The input and output signals of the analog sensor circuit are passed through the analog-to-digital converters and then the quadrature signal of the input excitation signal is generated through SDFT Bin-1, which can extract only fundamental frequency by filtering all other harmonics. The sliding DFT is a tuned filter that can filter out the high-frequency components, especially the second harmonics present in the correlated signal of the analog front end and the quadrature component of the excitation signal. The digitized correlated signal is passed to SDFT Bin-0, which can extract only the dc present in the correlated signal. The averaged output from SDFT Bin-0 is further utilized to calculate the capacitance (C) and resistance (R) values of the sensors under test. The proposed use case in designing the interface is applicable for one measurement at a time either C or R sensing. Experimental validation of the proposed use case has been carried out by field-programmable gate array (FPGA) implementation of the algorithm. FPGA implementation is validated for the C range spanning from 54.28 fF to 5600 pF. The maximum absolute relative (AR) percentage error observed is 1.27% for the measured range of 20–5600 pF and 1.92% for the measured range of 54.28 fF–10 pF. The maximum AR percentage error observed for R measurement in the range of 10 k $\Omega $ –4 M $\Omega $ is 1.157%. The application of the proposed interface includes smart devices, touch sensors, and automotive electronics.