Two operational amplifiers are connected in series. The first operational amplifier has a very stable drift characteristic and a feedback capacitor such that it has a very low frequency breakpoint. The second operational amplifier has a very fast response characteristic but may have a poor drift characteristic. A feedback impedance connects the output of a second operational amplifier to the input of the first operational amplifier and the input to the entire circuit is through an input impedance to the inverting input of both the first and second operational amplifiers.