An analog-to-digital converter includes a successive approximation converter, a voltage comparator, and a controller. The successive approximation converter receives an analog input voltage and a first reference voltage, determines the level of a voltage of a first node as a negative level of the analog input voltage, and using a successive approximation method determines an output logic value corresponding to one bit of the N-bit output digital code at every one successive approximation cycle while adjusting the level of the voltage of the first node based on a level of the first reference voltage. The voltage comparator compares the level of the voltage of the first node with a level of a second reference voltage to generate a comparison logic value. When the output logic value or the comparison logic value satisfies a condition, the controller terminates conversion and determines the N-bit output digital code.