A successive approximation ADC includes a switch circuit, a conversion circuit, a comparison circuit and a controller. The conversion circuit outputs first and second comparison voltages that are respectively equal to first and second input voltages when the switch circuit operates in an ON state. The comparison circuit compares the first and second comparison voltages to generate first and second comparison signals. Generation of least significant bits of the first and second comparison signals is related to two first capacitors and two second capacitors of the conversion circuit. When the other bits of the first comparison signal have identical logic values, the controller provides two different voltages respectively to the first capacitors and respectively to the second capacitors; and when otherwise, the controller provides one of the voltages to both of the first capacitors, and provides the other one of the voltages to both of the second capacitors. |