A SAR ADC including a comparator, an input switch unit, a positive conversion capacitor array, a negative conversion capacitor array, and a SAR controller is provided. The input switch unit alternately couples and decouples a differential analog input signal to the comparator. The positive and negative conversion capacitor arrays sample the differential analog input signal during the sampling phase. The SAR controller resets the switches in the capacitor arrays at the end of the sampling phase to change the sampled voltage into a residual signal, generates an intermediate digital code to control the switches during the conversion phase according to an output of the comparator to convert the residual signal to the intermediate digital code, generates the digital code according to the intermediate digital code, and uses an inverted intermediate digital code to control the switches at the end of the conversion phase.