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OPA862: A1 Input Bias Spec

joel mock
Prodigy 60 points
Part Number: OPA862
Other Parts Discussed in Thread: OPA855, OPA846

OPA862 application includes photodiode amplifier.  However, data sheet indicates an input bias of IB Input bias current, A1 =  3.1 µA.  That seems unusually high for this sort of app. Is there an error in the units.  Should this parameter be 3.1 nA instead of uA.  Thanks, Joel

  • +1
    TI__Expert 8640 points

    Hi Joel,

    The uA unit for the bias current of the A1 amplifier in the device is correct. Another way to confirm certain device parameters is by looking to see if the datasheet has the graphs for that specific parameter, which in this instance they are.

    Best Regards,

    Ignacio

  • +1
    Prodigy 230 points

    Hi Joel,

    You are right, that it can be surprising. In that high speed photodiode application the current range is 0mA to 5mA, that is why the feedback resistor has a value of 1k. So the 3.1 uA means negliible DC contribution (less than 0.1%), in AC applications does not even matter.

    I hope it can help to understand why such a relatively high bias current can fit to high speed photodiode applications where the diode current can be well above 1 mA (e.g. in optical fiber transceivers). Of course, it is useless if the photocurrent is in the uA range.

    Best wishes, Zoltan

  • 0
    Guru 140200 points

    Hi Joel,

    the OPA862 is a HF-OPAmp and in HF-TIA-applications higher input bias currents are usually tolerated, because often only the AC content of photodetector signal is used. Offset voltages caused by the voltage drop of input bias current across the feedback resistor of TIA play a minor role then and become only relevant when the output voltage swing is going to be limited. See the OPA846 and OPA855, for instance, which show even higher input bias currents.

    And, when comparing with the OPA846 and OPA855, it is also fair to say that the OPA862 offers a high input impedance. Of course, the source impedance must not be all too high, because otherwise the input bias current of OPA862 may limit the input voltage range.

    Kai