Qualification of a stable capacitive sensor interface based on capacitance-resistance comparison

This paper presents the principle of operation and the qualification results of a thermally stable high-precision capacitive sensor interface (CSI), which is based on a capacitance-resistance (C-R) comparison. An excellent performance of the CSI is demonstrated due to the high-accuracy and high-stability resistive reference implemented. A dedicated measurement set-up is built and a systematic measurement strategy is proposed to qualify the interface. The measurement results show a thermal stability better than 2.5 ppm/°C in all measurement ranges (5 pF, 10 pF, 20 pF, 50 pF, and 100 pF). Applying offset cancellation improves the thermal stability even further to 1.8 ppm/°C. The measured maximum deviation of the transfer characteristic from a linear one is 0.0023% for all measurement ranges. The dynamic range for 200 μs conversion time is extended up to 19 bits with the help of the “zoom-in” technique.