Hi all, I'm a new graduate student starting to work in a nanofab lab and most of my learning comes from digging around the web and reading research papers. I am trying to do the following and was hoping to get some feedback/advice...

I am trying to measure the current for five FETs simultaneously using the NI BNC-2090A.

My current setup allows me to measure one device by using the following setup:

My gate voltage and bias voltage is controlled by the NI BNC-2090A (which is connected to the DAQ card on the computer). I read the current using a Keithley 617 which is connected to the computer using GPIB-USB. Everything is controlled in labview.

What I would like to have set up is to use the 2090A to control the gate and bias but also use it to read the input and output current from the transistors under test. The advantage is that I can read 5 currents simultaneously and also much faster than the Keithley 617. I was told I would need op-amps for this setup because the 2090A only reads voltage so the opamp would allow a conversion from current to voltage.

My questions are... Opamps take in a voltage and amplifies it, why was an opamp recommended for this use? The range of current which I would need to measure goes from the picoamp range to microamps. I would need to get 5 opamps to achieve a current-to-voltage conversion from 5 devices. When shopping for opamps, what factors would need to be consider when trying to convert a picoamp to a voltage? Another restriction would be the current must covert to a voltage ranging from 0-10V as restricted by the 2090A but I assume this is solved though tweaking the gain.

I have limited experience in the field of electronics. Any feedback/thoughts would be very helpful! Thank you.

elionix wrote on Sep 13^th, 2010, 7:16pm:


The opamp circuit can be configured as a "current to voltage converter" like this:


In your case the photodiode would be replaced by your FET. I'd include a big cap across the 5M resistor to filter noise. You'll need something more complex than what the figure shows... I think you are going to need to learn a lot about electronics given that you want to measure across 6 orders of magnitude (picoamps to microamps)... hopefully you mean 100s of pA.

You'll probably need a MOS or JFET opamp to get the current noise down. Look around at sites like National, Analog Devices, Texas Instruments and you might find some application notes that will help.

This isn't the best forum... The folks here are more into building integrated circuits than using discrete opamps.

This note from Keithley might help you see what you are up against. http://www.keithley.com/data?asset=6169

Best of luck,
Rob