An analog-to-digital converter includes a comparator configured to compare an input signal with a reference signal and to output a comparison signal indicating a corresponding comparison result, a control logic configured to output a control signal for adjusting the reference signal based on the comparison signal, and a reference signal adjusting circuit configured to adjust the reference signal based on the control signal. The comparator includes a first pre-amplifier configured to amplify a difference between the input signal and the reference signal using a first transistor having a first size, a second pre-amplifier configured to amplify the difference between the input signal and the reference signal using a second transistor having a second size different from the first size, and a latch configured to generate the comparison signal using at least one of an output of the first and second pre-amplifiers. The first and second pre-amplifiers share the latch.