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

[Russ Ramirez]

I'll only add two things. You can believe Keithley's specs for equipment
made prior to their acquisition by Danaher in 2010 or so. You also have to
make these measurements under controlled conditions; for example, but using
a Triax connected Faraday cage, not ever touching what you are measuring
before you measure it, etc. As Ed mentioned, there is good info out there
by Keithley on the subject for guidance.

Russ

On Fri, Jun 5, 2015 at 11:46 AM, Ed Breya <eb at telight.com> wrote:

> 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
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