A Switched-Capacitor Parallel <i>R–C</i> to Digital Converter for Leaky Capacitive Sensors

A novel Parallel R-C to Digital Converter (PDC) that can accurately measure the capacitance independent of the leakage resistance of a capacitive sensor is presented. The proposed PDC can also quantify the leakage resistance associated with the capacitive sensor. There are several techniques reported for measuring the capacitance of a leaky capacitive sensor. Nevertheless, the majority of these methods are AC excited, necessitating amplitude and frequency stability for precise measurement. The proposed PDC is a DC-excited converter that can measure the capacitance in a single measurement cycle while canceling the effect of parallel leakage resistance, thereby attaining improved conversion time. The developed circuit is less complex, provides a direct digital output, and has the robustness of DC excitation. The proposed PDC can measure the resistance particularly low value with a predefined minimum ratio of resolution to absolute value. Furthermore, the converter relies on dual slope, inherently offering strong noise immunity. Additionally, the proposed converter is made insensitive to the stray capacitances at the sensor terminals. The developed prototype of the PDC exhibited a capacitance measurement range of 3 pF to 1500 pF with a worst-case linearity error of 0.16% even with leakage resistance as low as 50 kΩ. The prototype gives resistance measurement range of 100 Ω to 100 kΩ with a worstcase linearity error of 0.12%. Cross sensitivity of capacitance measurement gives a worst-case relative error of 0.98% and for resistance measurement is 0.03%. The prototype results show the efficacy of the novel converter towards measuring wide range parallel R-C sensors, particularly capacitive sensing applications with leakage resistance and low value resistance measurements with parallel capacitance.