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

[Ed Breya]

Yes, the input DC bias current can be compensated for, as long as it 
doesn't change too quickly, or isn't too big to make the input amplifier 
circuits run out of dynamic range. In a digital meter environment, all 
sorts of auto-zeroing and multi-sloping things and digital signal 
processing can be going on to correct for various device and circuit 
limitations, which would have been very difficult or impossible in the 
old analog machines.

Even in analog, some correction can be implemented fairly easily. A lot 
of the old Keithley electrometers have controls called "zero 
suppression," which can effectively offset the bias current, or even 
much larger amounts up to hundreds of full-spans, for certain 
applications. It's not quite the same as having true zero bias current - 
you still have to be aware of the effects and the compensation settings, 
depending on the measurement.

I modified my Keithley 417 by replacing the original zero suppression 
last digit fine adjust pot with a ten turn helipot and kilodial knob, 
which provides higher resolution. I can dial in the 17 fA bias current 
offset directly. When I eventually get around to washing and silicone 
coating the electrometer tube, I expect to get it down to around 2-5 fA.

My Kethley 410 electrometer doesn't have zero suppression - just a 
"zero" knob to set the input offset voltage over a small range to zero 
the meter. I added a bias nulling circuit which adds a small variable 
offset voltage superimposed on the output signal to the highest feedback 
resistor (E11 ohms) , with an optocoupler in PV mode. The effective bias 
current can be set to zero on the last two or three most sensitive 
ranges. On the other ranges, it doesn't matter since the effect is so small.

Ed