A CMOS interface for differential capacitive sensors using a time-to-digital converter

A high-accuracy CMOS interface for differential capacitive sensors using a time-to-digital converter (TDC) is presented for high-accuracy ratiometric operation. A capacitance-to-time converter (CTC) is developed based on a switched-capacitor (SC) sample/hold (S/H) circuit and an integrator. The proposed TDC achieves high resolution without using higher frequency clock signals. The Performances of the proposed CMOS interface are simulated by HSPICE. Simulated results have demonstrated that the proposed interface can convert the difference-to-sum ratio of sensor capacitors to digital numbers. The resolutions of 11.3 bits are achieved with calibration. The maximum nonlinear errors are smaller than ± 0.55 LSB. A prototype circuit built using discrete components has confirmed the principles of operation.