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 <inline-formula> <tex-math notation="LaTeX">${C} _{x}$ </tex-math></inline-formula> and resistor <inline-formula> <tex-math notation="LaTeX">${R} _{x}$ </tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">${R} _{x}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${C} _{x}$ </tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">$\Delta {R}$ </tex-math></inline-formula> and capacitance <inline-formula> <tex-math notation="LaTeX">$\Delta {C}$ </tex-math></inline-formula> can be very small compared with the nominal values <inline-formula> <tex-math notation="LaTeX">${R} _{0}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${C} _{0}$ </tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">$\Delta {R}$ </tex-math></inline-formula> in <inline-formula> <tex-math notation="LaTeX">${R} _{x}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\Delta {C}$ </tex-math></inline-formula> in <inline-formula> <tex-math notation="LaTeX">${C} _{x}$ </tex-math></inline-formula> 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 (<inline-formula> <tex-math notation="LaTeX">${R} _{0}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${C} _{0}$ </tex-math></inline-formula>) 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 <inline-formula> <tex-math notation="LaTeX">${R} _{0}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${C} _{0}$ </tex-math></inline-formula>. The results show that the output of the proposed digitizer is independent of the <inline-formula> <tex-math notation="LaTeX">${R} _{0}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${C} _{0}$ </tex-math></inline-formula> and can accurately measure the <inline-formula> <tex-math notation="LaTeX">$\Delta {R}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\Delta {C}$ </tex-math></inline-formula> without any cross-sensitivity.