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OPA322: Pico Ampere Measurement

Stephan Mauerer
Prodigy 10 points
Part Number: OPA322
Other Parts Discussed in Thread: LMC6001

Hello,

i want a design where i can measure 1 Channel in a Range from 1pA ..... 1µA with 1pA Resolution and <1pA Accuracy. Is the opamp OPA322 sufficient for this application and which outside components should be used.

Or which alternate part (AFE Frontend / ADC) can i used for this a application?

Many thanks and best regards

Stephan

  • Former Member
    0 Former Member

    Hello Stephan,

    Without knowing much about the application, I would recommend two things:

    1. With such a small current, you'll probably want to use a current integrator, or charge amplifier.  In other words, the current is very small so you measure it by observing the time it takes to charge an amplifier.  There are lots of resources to help you get started.  Here is a previous thread on the topic.  We also have an application note and an analog journal article on the subject.

    2. Make sure to pick an amplifier with a low bias/input current.  You don't want the parasitic effects of the amplifier to ruin your measurement.  You'll want a part that is rated for fA.  Also, the bias current can change a lot across temperature.  This is especially true of CMOS devices once you reach the higher temp regions.  So, don't forget to consider this.

    If you are not working at an extreme temperature, then the LMC6001 may be a good option.

    Please let me know if you need any further assistance.

    Regards,
    Daniel

  • 0
    Guru 140200 points

    Hi Stephan,

    assume you have an ADC with 5V full scale input voltage and a TIA is used to transform the measuring current into a voltage. Then a 5V / 1µA = 5M feedback resistor would be needed in the TIA. A measuring current of 1pA would cause an output voltage of TIA of 1pA x 5M = 5µV. So you will need an OPAmp for the TIA with less than 1pA input bias current and less than 5µV input offset voltage.

    The OPA322 shows an input bias current of up to 10pA and an input offset voltage of up to 2000µV, both at room temperature. So, no, the OPA322 could not be used for this task.

    By the way, you would need an ADC providing a precision of

    log (1µA / 1pA) / log 2 = 19.9 bit.

    Kai

  • Former Member
    0 Former Member in reply to kai klaas69

    Hi Kai,

    I would think the noise due to the 5MOhm resistor would overwhelm the accuracy.  A quick and rough estimate gives me a noise RMS value that is a couple times larger than the offset of the amp.  That being said, I also assumed a decently wide bandwidth.  I suppose he could cut that down.  But then with so little bandwidth he might not be able to drive the ADC from that amp.

    Just a thought.  Maybe I have missed something.

    Regards,
    Daniel

  • 0 in reply to Former Member
    Guru 140200 points

    Hi Daniel,

    I even haven't thought about noise up to now...

    Kai

  • Former Member
    0 Former Member

    Hi Stephan,

    Since it has been a while since this thread has had any new activity, I am going to mark it as resolved.

    If you have any further questions, feel free to come back and ask.  The thread will lock after a further period of inactivity.  If that occurs and you still have a question, then feel free to come here and press "+ Ask a related question" at the top of the screen.

    Regards,
    Daniel