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TIA gain selector for low current measurement

Albert Alvarez Carulla
Prodigy 110 points
Other Parts Discussed in Thread: DDC114, DDC316, INA116, LMC6001, TIPD104

Hello,


I'm working into an application to measure low levels of current (1pA or less) and I don't find a solution to implement a variable gain to achieve different ranges of measurements (1pA-1uA). I have found some solutions that have current leakage or quiescent current (multiplexers) as a drawbacks. Anyone know a good solution to achieve it?


Thank you very much for your time.

 

  • 0
    TI__Genius 14825 points

    Albert,

    Are you trying to measure current to a load or current from a source like a photodiode?  If you are looking for a current shunt monitor that measures pA, we don't have that.  You might be able to make something with a PGA and an INA, like http://www.ti.com/tool/TIPD135.  To measure small currents from things like photodiodes, perhaps the DDC series could be what you are looking for.  Check into the DDC114 or the DDC316 to start off, as they have different current measurement ranges and measurement speeds.


    Jason Bridgmon

  • 0
    TI__Mastermind 25825 points

    Hello Albert,

    As you stated, for 1pA currents you need devices with ultra-low input bias currents (e.g. INA116, OPA129, LMC6001). For such a large load current range you need to change the gain so you don't violate the linear operating region of the device.

    If you can place your sense resistor on the low side, your common-mode voltage is ~0V. Of the three aforementioned devices, I believe an implementation using the LMC6001 is the most straightforward. You could place the device is a non-inverting configuration on the low side and use a switch to change the gain. The general idea is outlined in TIPD104, though that design uses a unique instrumentation amplifier with too much input bias current. The other devices I mentioned would require dual supplies and the PCB layout may be more difficult.

    If you implement such a solution, we would greatly appreciate it if you can share your results and recommendations.

  • 0 in reply to Pete%20Semig
    Prodigy 110 points

    Ups! Sorry for I have not answered before. I have not received an email notification about the replies. Sorry again.

    Finally I'm going to use a rotary switch for gain selector. I will share with you the results as soon as it has implemented ;)

     

    Thank you very much for your replies and sorry again for the delay of mine.

  • 0
    Prodigy 50 points

    Having wires to switches with pA-level signals will probably cause pickup of weird signals. I'd recommend using a transimpedance amplifier with LMC6001 and small shielded reed relay to switch the feedback resistor. 

    Also, I recommend reading the What's All This Femtoampere Stuff, Anyhow ? by Bob Pease, to get to know some of the problems that you'll encounter when working with small currents. Nanoamps and microamps are huge, you know!