Femtoampere measurement help need, please

[juaco]

Hi guys

I'm designing a current amplifier from 100fA resolution to 1uA full scale.
Any information like better topology, links to papers, personal experience, layout techniques, problems, tips and tricks will be welcome.

Thank you in advance.
Juaco

 

[terryssw]

femtoampere? If is possible, or practical? thermal noise already have several nA!!

 

[juaco]

Yes, it is possible .... if you have enough time to measure it.
I forgot to add that the settling time for a 1pA measurement must be 1 second, hence the noise could be filtered somewhat.
The accuracy shall be better than 10% with a 5% repeatability.

Also, I found that there are too few OPAmps that could do the job:
OPA128LM (not for new designs )
OPA129V
AD549LH
LMC6001

Here are some usefull links that I found, about practical issues
What's All This Teflon Stuff, Anyhow?
**broken link removed**

Other problem with these design is the high dinamic range: 100 fA to 1uA