A passively compensated capacitive sensor readout with biased varactor temperature compensation and temperature coherent quantization

This paper presents a frequency-mode capacitive sensor readout front-end operating in a wide temperature range. First, a biased varactor temperature compensation (BVTC) is proposed to compensate the aggregate temperature gradients from the sensor and the oscillator circuit, achieving a nullified temperature coefficient for the oscillation frequency. Second, a temperature coherent quantization (TCQ) approach is proposed to enhance the sensor' sensitivity and to provide a self-referenced clock for digitization, whereby the influence from the temperature effect of the clock is minimized through a hybrid down-conversion time-to-digital converter (DTDC). A prototype chip was fabricated using the 0.18-μm CMOS process and it was verified using a commercial 5.92-to-6.53-pF capacitive pressure sensor over a temperature range of −20°C to 120°C. Proven by experiments, the prototype presented as good as ±0.4% full-scale pressure error in the 140°C temperature range.