An analog-to-digital converter, configured to convert an input signal into an n bits digital output signal, includes a capacitor module, a control signal generation unit, a comparator, and a register. The capacitor module is configured to receive the input signal at a sampling phase in a normal mode, and to generate a first sampling signal and a second sampling signal according to the input signal in a conversion phase. The control signal generation unit is configured to adjust the first sampling signal or the second sampling signal in the conversion phase. In the normal mode, the comparator is configured to compare the first sampling signal and the second sampling signal in the conversion phase to generate n comparison signals. The register is configured to store the n comparison signals as the digital output signal, and output the digital output signal in the normal mode.