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A novel readout integrated circuit (ROIC) with tunable sensitivity, variable resolution, and especially multiple and selectable input ranges is designed, fabricated, and characterized for various capacitive MEMS sensors. The ROIC was fabricated by using 0.35 mum 3.3 V CMOS process and consisted of a capacitance-to-time (C-T) converter, a high speed signal counter, a phase-locked loop (PLL), control logics and synchronous peripheral interfaces (SPI). By using SPI, the clock frequency (fCLK) of the ROIC can be changed from 10 MHz to 160 MHz, which results in resolutions up to 14 bit. The measured minimum detectable capacitance of the ROIC were 2.86 fF for fCLK = 10 MHz and 180 aF for fCLK = 160 MHz, which were equivalent to 9 bit and 14 bit resolutions, respectively.