The invention relates to an analog-to-digital converter comprising a reference voltage generating circuit, two coarse/fine comparators and two encoders for encoding the comparison result of the two coarse/fine comparators. In the invention, the two coarse/fine comparators processes a coarse comparison procedure and a fine comparison procedure on an input voltage in different clock cycle, thus, a sampling voltage error caused by an error of sampling time decreases. In another aspect of the invention, the capacitance of the input capacitor of the analog-to-digital converter decreases because the comparators for coarse comparison and fine comparison are the same, thus, a large power amplifier is not required for driving the input capacitor.