Patent ID: 7649480

Claim:
A circuit for obtaining a calibrated output signal from an input signal, the circuit comprising: an analogue gain amplifier for applying a variable gain to said input signal; an analogue to digital converter for converting said input signal to digital form after amplification by said gain amplifier; a correction circuit for receiving a signal derived from the output of the analogue to digital converter and outputting a digital offset signal; 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, such that said correction circuit and said digital to analogue converter form a feedback loop applying an offset to said input signal, wherein 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 wherein said inverse gain is applied so as to establish a combined gain response applied around said feedback loop within predetermined bounds; and a controller for controlling said inverse gain circuit so as to vary the inverse gain automatically so as to maintain the loop gain factor within said predetermined bounds as the gain of the gain amplifier varies, wherein said controller is arranged to vary the gain of the inverse gain circuit more coarsely than the gain of the gain amplifier and wherein the circuit is operable such that said variation of the loop gain factor around the circuit's feedback loop is restricted within said pre-determined bounds, wherein said pre-determined bounds enable the feedback loop to maintain stability.