There is disclosed a calibration method for an A/D converter. The A/D converter includes a first amplifier to amplify first and second voltage signals, a second amplifier to amplify the first and second voltage signals amplified by the first amplifier, and a comparator to compare the first and second voltage signals amplified by the second amplifier. The calibration method performs short-circuiting input ports of the second amplifier, comparing the first and second voltage signals inputted to the comparator to obtain a first result, calibrating output voltage of the second amplifier according to the first result, short-circuiting input ports of the first amplifier, opening the short-circuited input ports of the second amplifier, comparing the first and second voltage signals inputted to the comparator to obtain a second result, and calibrating output voltage of the first amplifier according to the second result.