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OPA227 linearity problem

Roelof Polman
Prodigy 30 points
Other Parts Discussed in Thread: OPA227, OPA627, OPA277, OPA177, OPA209, OPA211, OPA172, OPA192

Found a non-linearity when using the OPA227 in -1x mode, 2kOhm resistors

Opamp seems to oscillate @ 100MHz

Supply -15 / +15V, perfectly decoupled.

Oscillating when the output is driven negative, about -3 .. -9

Oscillation occurs when there is a relative high load, about 5 mA

Stops when a small capacitance ,10pF or oscilloscope probe, is connected to the output.

Resulting error is about 20 ppm linearity error on negative side.

Same circuit with a FET opamp OPA627 or oipa131 is OK

But these FET opamps are more noisy and less DC-stable.

Seems the output transistor is oscillating stand-alone, signal is detected in the bipolar input

Questions :

Other persons found this behaviour ?

Is there a better bipolar opamp for this application.

  • 0
    TI__Guru 64530 points

    Roelof,

    We have previously encountered this problem with OPA227 in certain application where a small capacitance at the output eliminates the small-level oscillation.  A sure fix is to use a snubber circuit - 20ohm resistor in series with 500pF placed between the output and ground.  Alernatively, please use OPA277 instead OPA227.

  • 0 in reply to Marek Lis
    Prodigy 30 points

    Hello Marek Lis,

    thanks for the confirmation.

    The OPA177 is not an alternative for us, we need the speed of the OPA227.

    The OPA227 is a relative old product, I hope, there is a newer device ( bipolar and -15V/+15V supply) with the same or better specs for DC and AC.

    Testing takes a lot of time, because not all devices show this problem in my test.

    regards,

    Roelof

  • 0 in reply to Roelof Polman
    TI__Guru 64530 points

    Relof,

    We do have newer bipolar op amps available with higher speed and lower noise:  OPA209 and OPA211.

    Also, please take a look at the recently introduced high voltage CMOS op amps with the speed, input offset and drift comparable to OPA227: OPA192 and OPA172.

  • 0 in reply to Marek Lis
    Prodigy 30 points

    Hello,

    I think the OPA209 is the best alternative to the OPA227,

    I will try a few.

    Can I be sure, these suggested opamps do NOT have this problem?

    Again, thanks for you help,

    Roelof Polman

  • 0 in reply to Roelof Polman
    TI__Guru 64530 points

    Roelof,

    The parts I suggested, including OPA209, are designed on TI modern process and use completely different circuit topology than the one used in OPA227.  For that reason, you can rest assure that they do not have the problem associated with OPA227.