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OPA4991-Q1: Maximum limit of feedback resistor value for voltage follower circuit

MESH
Intellectual 840 points
Part Number: OPA4991-Q1
Other Parts Discussed in Thread: OPA4991

Hello,

I am designing OP AMP circuit that circuit uses OPA4991-Q1.

I will use OPA4991-Q1 for voltage follower circuit under below conditions.

IN+ pin : Connect 4/5 voltage divider resistors (100kohm and 25kohm)
* Input signal for voltage divider circuit : 20Vp-p 1kHz sine wave signal (Offset voltage: 0V)

IN- pin : Connect OUT pin through 20kohm feed back resistor.
* To cancel out effect of input offset current effect : value of 100kohm // 25kohm = 20kohm


I would like to confirm maximum limit of feedback resistor value for voltage follower circuit using OPA4991-Q1.

I am concerned that too large feedback resistor value will affect the operation of the OP AMP circuit.


Regards,
MESH

  • +1
    Guru 48385 points

    Morning MESH

    The quad OPA4991 is a CMOS input device, so bias current cancellation is not effective. Putting in 20kohm feedback is just increasing your output noise. Perhaps no more than 100ohm feedback would be better. 

  • +1
    TI__Mastermind 26471 points

    Hey MESH,

    I'd like to add on to Michael's comment here. I've copied the bias current spec from the OPA4991-Q1 datasheet. Note that your input bias current is a +/- value.

    Your bias current is just as likely to be positive as it is negative, and will be random. Even two pins of an input stage can have input bias current in different directions, so instead of cancelling out your Ib error, you could potentially double it. At room, your error would be +/-200nV of DC offset. This would be worse at high temp, but still incredibly small even compared to the offset error.

    If you want a voltage follower circuit, I would recommend just not adding a feedback resistor.

    Best,
    Jerry

  • 0 in reply to Michael Steffes
    Intellectual 840 points

    Hello, Michael.

    Thank you for your reply.

    I will remove feedback resistor for my circuit.

    Regards, MESH

  • 0 in reply to Jerry Madalvanos
    Intellectual 840 points

    Hello, Jerry

    Thank you for your reply.

    My knowledge of bias currents has increased.

    Regards, MESH