An analog-to-digital converter including a comparator, a control module, a voltage adjusting module, and an evaluating module is provided. The comparator compares an analog input voltage with a feedback voltage and generates a comparison result. Based on the comparison result, the control module generates a control signal. The voltage adjusting module increases or decreases the feedback voltage toward the analog input voltage according to the control signal. The voltage increase amount and decrease amount provided by the voltage adjusting module are corresponding to a first digital value and a second digital value, respectively. The evaluating module generates the first digital value and the second digital value based on the control signal. According to the first digital value and the second digital value, a digital signal corresponding to the analog input voltage is generated.