A Switched-Capacitor Circuit-Based Digitizer for Efficient Interfacing of Parallel R-C Sensors

In this paper, a new digitizer, which accurately provides the values of the change in the capacitor Cx and resistor Rx of a parallel R-C sensor, is presented. Conventional measurement schemes for parallel R-C sensors require a precision sinusoidal excitation, followed by a signal conditioner, a demodulator and an ADC. This increases the cost and complexity of the scheme. Hence, a dc excitation and an ADC modified to measure Rx and Cx directly is preferred, by the design engineers. But, such a scheme has not been reported. Another challenge is the low-modulation nature of such sensors, i.e., the change in resistance ΔR and capacitance ΔC can be very small compared with the nominal values R0 and C0 of the sensor. This introduces a large offset in the digital output, leading to an inefficient conversion. The proposed digitizer employs a dc excitation and provides accurate digital values of change ΔR in Rx and ΔC in Cx of the sensor. The digitizer employs a simple, but effective automatic calibration, with the help of an improved switched capacitor controlled voltage and obtains an output independent of the nominal values (R0 and C0) of the sensor leading to an efficient conversion compared with the existing schemes. A detailed analysis of the sources of errors affecting the output of the digitizer has been conducted. A prototype of the proposed digitizer has been developed and tested in the laboratory for a wide range of R0 and C0. The results show that the output of the proposed digitizer is independent of the R0 and C0 and can accurately measure the ΔR and ΔC without any cross-sensitivity.