Measuring femto amperes.

[William Gaatjes]

Very interesting articles on how to measure very small currents.

I am trying to make a circuit to measure currents with a resolution of 10nA.
I will be happy if i can reach 20nA or eeven 50nA resolution.
That is not easy at all while being limited in power supply and components usage. With opamps it is a lot easier to do, bit still quite the challenge.


But these are very informative articles.
It explains that in the femto Ampere range, even cosmic rays cause measurement errors. How movement of cables cause measurement errors because of the effects of friction between an insulator and a conductor.
Humidity is a more known cause of course.


http://www.edn.com/design/analog/4315417/Measuring-nanoamperes#


http://www.electronicdesign.com/files/29/14692/14692_01.pdf

 

[WhoBeDaPlaya]

I don't know about femto-amps, but we regularly deal with currents in the ~10nA range (Raith and Vistec e-beam writers).

 

[William Gaatjes]

Well, it was not an official design request at the moment, but i expect that it will be requested, henceforth i did some preliminary research...
But i kind of hope that 100nA resolution will be sufficient.
Otherwise, it will be a lot of electronic parts and smart software.

I need to build a current source that can go from mA to nA.
But in the nanoAmpere range it is better to actually measure the current to the load, since so much factors can influence the amount of current that actually flows, measuring will be required. Anyway, i will hear it soon enough. OpAmp with picoamps input bias current are available but require a lot of design care.

 

[William Gaatjes]

From the article i got an ic that can measure nA easily.
The DDC112 is a current input adc.

http://www.ti.com/product/ddc112

http://www.ti.com/lit/gpn/ddc112

By using a capacitor to store the to measure current in, even a very small current can be measured because a capacitor keeps charging until there is no more voltage difference between the capacitor plates. It just takes time for the charge to accumulate. When the capacitance is known and a voltage is present,
Then a charge is present and when the time is also known, the average current value can be determined.