Picoampere Input Current, Triple Op Amp Instrumentation Amplifier with Bias Current Cancellation

The upper three opamps form a standard instrumentation amplifier. The additional opamp at the bottom is used to provide a bias current into all the inputs so that the source signal loading is minimized. The circuit takes advantage of the close matching of sections within the IC so that when the lower section is bias-canceled, the other sections are reasonably well canceled as well.