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We present a low-power, low complexity, and wide-dynamic-range universal sensor readout circuit that converts the sensing capacitance or resistance changes to digital duty cycles based on pulsewidth modulation (PWM). The readout circuit utilizes a RC-controlled pulse generator and produces pulse signals whose duration width is proportional to the charging time. The circuit is comprised of complementary CMOS circuit that enables the proposed design to consume significantly less power compared with traditional designs. The proposed design is universal and can be configured according to the application requirements. The sensor interface chip is designed and fabricated in TSMC 0.13-μm CMOS technology. The proposed interface achieves 13-aF to 10.7-nF capacitance and 5-Ω to 11.5-MΩ resistance sensing ranges with 60- μW power consumption. The functionality of the full circuit, including circuit analysis, noise analysis and measurement results, has been demonstrated.