Flash analog to digital converter and calibration method

A flash analog digital converter includes double-differential comparator circuits and a calibration circuit. Each of the double-differential comparator circuits compares a first input signal with a corresponding one of a first set of reference voltages, and compares a second input signal with a corresponding one of a second set of reference voltages, in order to generate a corresponding one of first signals. Under a testing mode, the calibration circuit outputs a first testing signal as the first input signal, outputs a second testing signal as the second input signal, and calibrates a common mode level of each of the first and the second input signals or calibrates at least one first reference voltage in the first set of the reference voltages and at least one second reference voltage in the second set of the reference voltages according to a distribution of the first signals.