Abstract: An analog-to-digital converter is used to digitalize the pressure data in Battery-less TPMS (Tire Pressure Monitoring System). To reduce power consumption, an 8-bit low-power SAR(Successive Approximation Register) ADC, with a modified switching method, is designed after analysis the requirements of the interface of pressure sensor. The DAC switching method uses V_ref/2, rather than V_ref in conventional way, as the only reference voltage to digitalize the input signals with the amplitude range of (0, V_ref). The proposed design reduces the average power consumption in the DAC by 87.5%during digitization. Post simulation results show that at the power supply of 3 V and reference voltage of 1.5 V and with the sampling rate of 2 kS/s, the average dynamic power is 0.9 μW, and the ADC can digitize the input signal in a full-scale range of 3 V with ENOB of 7.6 bits and SNDR of 63.2 dB.It can work steadily with a temperature range of -40~125℃.The ADC has been applied to the Battery-less TPMS SoC and taped out in a 0.35 μm BCD (Bipolar-CMOS-DMOS) automotive electronic process.