Nanovoltmeter amplifier for low level voltage measurements

A new design of an offset correction system that employs a time varying resistance as a probe for detecting the sign and magnitude of the equivalent input offset of an operational amplifier in a series-shunt feedback configuration is proposed. The time varying sense resistor is implemented by a MOS for which the problem of charge injection is considerably reduced by resorting to a proper discrete time sampling strategy for offset error detection. Significant design results include the extension of the useful amplifier bandwidth from a few Hz up to about one hundred Hz with a gain boost from 201 to 1001. With the new approach, a residual offset in the order of a few tens of nV is obtained which allows us to classify the system as a nanovolt amplifier.