This simple circuit is a gain-of-1000 inverting amplifier. It will amplify submillivolt signals up to signal levels suitable for further processing. In almost all system applications, it is best to use as OUTPUT much gain as possible in the MAX420, thus minimizing the effects of later-stage offsets.
For example, if circuitry following the MAX420 has an offset of 5 m V, the additional offset referred back to the MAX420 input (gain = 1000) will be 5 p.V, doubling Fig. 3-4 the system"s offset error.