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 C_x and resistor R_x 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 R_x and C_x 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 AR in R_x and ΔC in C_x 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 AR and AC without any cross-sensitivity.