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OPA695: OPA695 mysterious resistors

John Larkin
Prodigy 130 points
Part Number: OPA695


On the OPA695 data sheet, figures 49 and 51 and 58, there are 20 ohm resistors in series with the non-inverting input. Why are they there?

  • 0
    TI__Genius 15975 points
    Hi John,

    The 20-ohm resistors on the non-inverting inputs are for de-Qing purposes meant to improve op-amp stability at high frequencies.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 130 points
    Thanks. Is there a reason why they are in those figures, but not in others? They will increase noise a bit. "Improve opamp stability" is a big red flag.

    Incidentally, fig 64 is mis-titled.
  • 0 in reply to John Larkin
    TI__Genius 15975 points
    Hi John,

    I don't think there is a specific reason other than while connecting the non-inverting input to a reference voltage or GND. For clarity, the 20-ohm resistor is a recommendation to improve op-amp stability at high frequencies. As such, the OPA695 should be stable at unity gain. However, some times, due to bad layout there is enough trace capacitance on the board to introduce a pole which could potentially throw the op-amp into instability. This series resistor is mainly intended to de-Q in such scenarios and normally recommended for op-amps with GBW>1GHz.

    Thanks. We will fix fig 64 in our next datasheet revision.

    Best Regards,
    Rohit