Looking to measure femtoampere using uCurrent

[Aneikei]

Hi all. I'm looking to measure femtoamps
(1 femtoamp = .001 picoamps). I believe the uCurrent can measure picoampers. However if daisy chaining two uCurrent's together would that in theory allow me the sensitivity to measure femtoamps? And if possible would there be too much noise?

Thank you for you insight and time.

[tszaboo]

Smallest range is 1mV/nA, so you have 1nV/fA output. And the MAX4239 on it has typical 1000 fA input bias. So maybe you should use a tool which is not designed to do one million times bigger things.
You wouldnt measure micrometers with this, would you?

[EEVblog]

You need this:


and you need to read this:
http://www.tek.com/sites/tek.com/files/media/document/resources/LowLevelHandbook_7Ed.pdf

You can't just casually measure femptoamps.

[Gyro]

You could try this thread too:

https://www.eevblog.com/forum/projects/picoammeter-design/

[Aneikei]

Thank you much for the head start gentlemen. It's much appreciated. This is fantastic stuff!

[David Hess]

The example shown in that discussion thread here can get close.  I used a similar circuit with an LMC6081 graded for low input current for better precision than the LMC662 but you can also buy the pre-graded LMC6001.  Offhand I do not know of any better parts.

Construction is critical.  For a one off, I would air wire the input side over a ground plane.  When we did this on a printed circuit board, the input side was air wired with the non-inverting input pin of the DIP package bent to be horizontal to the printed circuit board.

Noise will limit precision.  One way to get around this is to replace the high value transimpedance gain resistor with a capacitor to make an integrator and then make measurements over time.  Use a low capacitance relay to reset the integrator.  Robert Pease wrote a couple articles discussing this implementation:

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

https://www.ti.com/ww/en/bobpease/assets/www-national-com_rap.pdf