A method for calibrating a fixed gain amplifier configured as a front-end amplification stage of an analog-to-digital converter including sampling a calibration voltage with normal and inversed polarity and with the fixed gain amplifier bypassed and with the fixed gain amplifier connected. An actual gain value of the fixed gain amplifier is computed from offset corrected digital output codes generated from converting the calibration voltage. A gain correction value for the fixed gain amplifier can then be computed based on the ratio of the actual gain to the ideal gain. In another embodiment, a method for calibrating an analog-to-digital converter including a fixed gain amplifier, an input buffer and a modulator generates an offset correction value using normal and polarity inversed input samples. The offset correct value provides correction for at least offset errors in the fixed gain amplifier, the input buffer and the modulator.