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OPA847 Pspice model

Isaac Troyano1
Prodigy 90 points

Other Parts Discussed in Thread: OPA847

Hello,

we are using the OPA847 as a transimpedance amplifier to read out a photodiode but we are not getting the same results in the simulation and in the lab. We checked that the simulation and the results in the lab are correlated while applying input currents of about 1mA but our detector sinks currents of around 5-6uA pp under certain conditions and is at these low values where we need to work harder on the simulation to improve the circuit response.

Is there any limitation on the spice model of the OPA847 that makes our simulations at low input currents (5-6uA) not representative of the real circuit behavior?

Thanks in advance,

Isaac

  • 0
    TI__Genius 12631 points

    Hello Isaac, There should be no difference in the accuracy of the model. Couple of thoughts:

    1. With an input of 1mA, depending on your transimpedance gain you will have a large signal output whereas with 5-6uA you will probably have a small signal output. It is possible that there are board parasitics which may not be accounted for properly and these are showing up with a small input signal.

    2. With a relatively small signal, noise from external sources including your sensor could be overwhelming your readings.

    If you could provide a schematic along with a comparison of the test/sim results I may be able to provide more assistance.

    Samir

  • 0 in reply to Samir Cherian
    TI__Mastermind 22825 points

    Hi Samir and Issac,

    To add to Samir's comments:

    Keep in mind the ~20uA input bias current of the OPA847 at room temperature (typical). Hopefully, when you are considering the "output", you've factored in this input bias current (which is a DC term- not sure how you are measuring the output (DC + AC?)) which flows through your feedback resistor and will create an output offset voltage.

    You could cancel the effect of the input bias current by adding a resistor in series with your non-inverting input, leaving with you the "offset current" (<1uA) left-over.

    Regards,

    Hooman