[volt-nuts] How are femto-amps measured?

Jan Fredriksson jan at 41hz.com
Sat Jun 6 05:06:05 EDT 2015

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

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?

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