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transimpedance instrumentation amplifier

Scot B
Prodigy 40 points
Other Parts Discussed in Thread: LMP7721, INA116, LMC6044

I need to measure picoamps to nanoamps with significant common mode noise.  Is it possible to build a transimpedance instrumentation amplifier?  I remember seeing a circuit a long time ago where a researcher built a instrumentation amplifier with 3 discrete op-amps but with a feedback resistor on the two inputs so it was effectively a transimpedance instrumentation amplifier.  Actually I just found the diagram, I hope it is readable.  Any advice on constructing such a circuit and what components that might be recommended?

  • TI__Mastermind 25825 points

    Hello Scot,

    While I have never constructed such a circuit myself, I don't see why it wouldn't work. For the input stage amplifiers you will want low input bias current. the LMP7721 is an ultra-low input bias current amplifier (just 3fA) and has very low offset voltage (50uV typical). One of my colleagues authored the following article series on sensing low level currents...you may find it helpful:

    www.edn.com/.../Design-femtoampere-circuits-with-low-leakage-part-one


    A couple other options that may be helpful are the INA116 and the LMC6044. The INA116 is an ultra-low input bias current instrumentation amplifier, but you may have to calibrate out the offset voltage. Finally, the LMC6044 quad op amp may be worth a look. Of course if you build that circuit yourself you'll have to take the resistors into account (tolerance, noise, etc.). Careful consideration of the PCB layout should be taken as well. The aforementioned article series and the INA116 data sheet contain some helpful guidance.

    If you do build the circuit, we would greatly appreciate a follow-up post with your results and lessons learned. Thanks!