This paper presents a low power, compact, and low-complexity pulse-width modulation-based interface circuit for capacitive MEMS sensors. The circuit is designed using a ring oscillator, an RC controlled pulse generator with high-pass filter, and a self-tuning inverter comparator to produce pulse width, which is proportional to differential capacitance and independent of parasitic capacitance. The high-pass filter is utilized to reduce the bandwidth of noise sources. The circuit provides control over sensitivity, dynamic range, and nominal point for the capacitance measurement by selecting controlling parameters, such as resistance of the RC pulse generator, biasing voltage of the self-tuning inverter comparator, and a reference capacitor using digital control signals. The circuit provides high linearity with higher sensitivity and lower power consumption. The sensitivity of the circuit is 0.56 to 3.62 mu s/pF depending on the controlling parameters. The maximum dynamic sensing range is 22 to 270 pF depending on the controlling parameters. The interface circuit is designed and fabricated using the United Microelectronics Corporation (UMC) 0.18-mu m CMOS technology. It occupies an active area of 0.17 mm(2) and consumes 98 mu W. A capacitive MEMS-based pressure sensor is also connected with the interface circuit to measure pressure throughout the digestive tract. The sensitivity for pressure from 101 to 200 kPa is 60 ns/kPa and from 50 to 101 kPa is 23 ns/kPa.