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

Jan Fredriksson jan at 41hz.com
Fri Jun 5 09:22:21 EDT 2015

I saw the other day a Keithley 2636B SMU which has 0.1fA resolution @
10uV in a "normal" Keithley DMM box size. Keithley also have some
systems with external amps that go down even further, to tens of
atto-Amp regions. Even the old HP resolves down to 1fA.

It made me curious.

Dear list: What's the basic principle of such measurements?

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