The invention discloses an analog front end device includes a calibration unit and at least a conversion circuit. The conversion circuit includes a clamper, a multiplexer, an voltage buffer and an analog to digital converter. The clamper receives an image signal and resets the DC voltage level of the image signal to generate a clamped signal. The multiplexer receives the clamped signal and a test signal and outputs the clamped signal or the test signal according to a selecting signal. The voltage buffer amplifies the clamped signal or the test signal to generate a buffer signal. The analog to digital converter converts the buffer signal into a digital signal. The calibration unit generates a gain correction value according to the test signal and calibrates the gain offset of the digital signal according to the gain correction value.