An analog-to-digital (ADC) calibration apparatus comprises a calibration buffer, a comparator and a digital calibration block. Each reference voltage is sent to a track-and-hold amplifier as well as the calibration buffer. The comparator compares the output from the track-and-hold amplifier and the output from the calibration buffer and generates a binary number. Based upon a successive approximation method, the digital calibration block finds a correction voltage for ADC offset and nonlinearity compensation. By employing the ADC calibration apparatus, each reference voltage can be calibrated and the corresponding correction voltage can be used to modify the reference voltage during an ADC process.