An analog-to-digital converter includes a comparison signal generation unit and a control unit. The comparison signal generation unit determines a logic level of a comparison signal by comparing an input signal with a selected reference signal based on a switch control signal in a first comparison mode, and by comparing a difference voltage with a ramp signal based on the switch control signal in a second comparison mode. The difference voltage is generated based on the input signal and the selected reference signal such that a level of the difference voltage is lower than a fine voltage level corresponding to a voltage level of the selected reference signal in the second comparison mode. The control unit generates the switch control signal based on the comparison signal and a mode selection signal.