As described, the pico-femto amp meters are "pass-through", current goes in AND out, at 10uV burden voltage. They are explicitly not shunt and as far as I understand, can't be trans-impedance amplifiers either.
The very lowest leakage OP-amp I know of, LMP7721 is rated 3 fA typ, with a specified limit of ±20 fA at room temperature, ie much higher than the sub-femto. The best candidate discreet DIFETI know of, LS320, has a specified input leakage resistance of 100Gohm which would be equivalent to a gate current of 10fA at 10uV, ie numbers in the right ballpark. Still that does not explain the current pass through. So still it makes me wonder how they do it. Even the old HP 4140B (model fell out of my last post) has a specified resolution to one fA. The only possibility I can think of is to use selected FETs and let the test current flow through D-S and control the gate voltage to the specified D-S voltage drop and correlate/calibrate test current to gate voltage. I have a Keithley 6485 and guess that out of pure curiosity I'll have to make a tear down at some point :-) Any thoughts? _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
