An input buffer for an ADC is provided. The input buffer includes a receiving circuit and an impedance circuit. The receiving circuit is coupled between a power supply and a sample-and-hold circuit of the ADC, and receives an analog input signal and generating an analog signal. The impedance circuit is coupled to the receiving circuit, and selectively provides a variable impedance. When the sample-and-hold circuit of the ADC is operated in a first phase, the impedance circuit provides a small impedance, and when the sample-and-hold circuit of the ADC is operated in a second phase, the impedance circuit provides a large impedance.