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

[MARVIN]

A simplest model is RC integration. 

http://electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow 


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

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?