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

[Ed Breya]

With modern digital readout meters, this can be very misleading in terms 
of actual useable capability. In this case, using the specified highest 
sensitivity 100 pA range, and six digits of digital resolution, gives 
E-10 A/E6 or E-16 A, which is 100 aA for the last digit. But, looking at 
the noise and accuracy specs shows that it is swamped by these numbers. 
The noise can be reduced greatly by sufficient averaging over long 
enough time. The same issues are encountered in voltage measurement.

For good info on making very small current measurements, investigate 
electrometer technology of the good old days, especially from Keithley. 
Back in the 1960s - 1980s, electrometers typically used special tubes or 
MOSFETs that had bias currents in the fA region, along with high 
resistances that topped out at around E11 to E12 ohms (which is still 
about the limit of practicality for R).

Nowadays there are opamps available with bias current in the fA region 
at room temperature, but noise is still a limiting factor. For better 
absolute accuracy and drift performance needed for modern digital 
meters, the ranging resistors can be lower - probably E10 ohms or less, 
since the meters can measure and resolve much smaller voltages.

Ed