Automatic Offset Correction for Measurements in the Nanovolt Range

In this paper, the design of a new offset correction system employing a time-varying resistance as a probe for the detection of the sign and magnitude of the equivalent input offset of an operational amplifier in a series-shunt feedback configuration is presented. In order to considerably reduce the charge injection effects resulting from the switching of the MOS transistor that is used for the implementation of the time-varying resistance, we resort to a proper discrete time-sampling strategy for offset error detection. With respect to a previous topology, the new approach allows us to extend the useful amplifier bandwidth from a few hertz up to about 100 Hz with a gain boost from 201 to 1001. With the new approach, a residual offset on the order of a few tens of nanovolts is obtained, which allows us to classify the system as a nanovolt amplifier.