An analog-to-digital converter includes a sample-and-hold amplifier (SHA), a plurality of stages of analog-to-digital converting (ADC) units and an adjustment circuit. The SHA samples an analog input signal. The ADC units serially converts the analog input signal sampled by the SHA into corresponding digital values, in which a first stage of the ADC units generates the digital value including most significant bits. The adjustment circuit controls the SHA by checking the most significant bits from the first stage of the ADC units to modify offsets of the analog input signal with respect to a reference value. A method for adjusting an analog input signal in an analog-to-digital converter and a method for converting an analog input signal into a digital signal are also disclosed herein.