A calibration circuit and method suitable for black level calibration in image processing, the circuit comprising an analogue gain amplifier, an analogue to digital converter; a correction circuit for receiving a digital signal and providing a digital offset signal; and a digital to analogue converter for receiving said digital offset signal and feeding a corresponding analogue offset signal back to the input of said gain amplifier. The calibration circuit is arranged such that the correction circuit and said digital to analogue converter form a feedback loop applying an offset to said input signal and said correction circuit includes an inverse gain circuit for applying an inverse gain to a signal within said correction circuit prior to said digital to analogue converter. Preferably the inverse gain applied is such that the total loop gain does not deviate too far from unity.