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OPA552: Unstable output

David__
Genius 3585 points
Part Number: OPA552
Other Parts Discussed in Thread: OPA551

I use 2 OPA552P. One is configured as a non-inverting amplifier.

The other one is configured as a unity gain buffer. On the breadboard, all OPA552 is connected to + - 30V power supply with a 0.1uF 50V X7R ceramic decoupling capacitors.

A periodic (100Hz) signal is amplified from the first OPA552 and it is connected to the buffer. However, the buffer's output is not stable, showing oscillation.

Whereas, the first amplifying Op Amp doesn't show this problem.

I used Metal Film resistors, by the way.

Even though I lower the amplification, this still happens.

What seems to be the problem? I can't find the reason to observe this unstable oscillation.

  • 0
    Guru 140200 points
    Hi David,

    have you read section 8.2.2.3 of datasheet? The OPA552 is intended for applications with signal gains of 5 or greater!

    Kai
  • 0 in reply to kai klaas69
    Genius 3585 points

    kai klaas69 said:
    The OPA552 is intended for applications with signal gains of 5 or greater!

    OMG, thanks. Kai. I totally missed that part.

    I also noticed this from the first page of the datasheet; The OPA551 is unity-gain stable and features slew rate (15 V/μs) and wide bandwidth(3 MHz)

    and the solution; using an external compensation technique in an inverting configuration.

    Too bad I don't have nano and pico capacitors.

    Then, switching my buffer Op Amp to OPA551 will solve this?

    I really appreciate your help.

  • 0 in reply to David__
    TI__Guru* 93105 points
    Hi David,

    Switching to the OPA551 which is a unity gain stable op amp should resolve the stability issue. Keep in mind that even a unity-gain stable op amp can become unstable if its output is loaded with a capacitance beyond the particular op amp's capabilities.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Genius 3585 points

    I deeply appreciate your help. One last question before closing, please.

    Thomas Kuehl said:
    Keep in mind that even a unity-gain stable op amp can become unstable if its output is loaded with a capacitance beyond the particular op amp's capabilities.

    Then how can I prepare this type of problem? My unity gain buffer output, using OPA551, will be connected to an unknown load so I wish to hear your advice.

    [snoa474a] Will something like this small feedback capacitor help to solve the aforementioned problem?

    If there are extra reading materials or general tips, I'm eager to listen, please.

  • 0 in reply to David__
    Guru 140200 points

    Hi David,

    what do you exactly mean by "unknown load"? The OPA551 cannot drive arbitrary capacitive loads. This is true for the very most OPAmps.

    Kai

  • 0 in reply to kai klaas69
    Genius 3585 points

    kai klaas69 said:
    what do you exactly mean by "unknown load"? The OPA551 cannot drive arbitrary capacitive loads.

    IRB(Institutional Review Board) approved human skins will be the load.

    I'm curious whether the OPA551 can drive whether the skin is contacted as a load.

  • 0 in reply to David__
    Guru 140200 points
    Hi David,

    you know that it can become very dangerous to touch the human body with electronics? Special safety regulations must be fullfilled then!

    Kai
  • 0 in reply to David__
    TI__Guru* 93105 points
    Hello David,

    The Improved Howland Current Pump utilizes both negative and positive feedback and is more difficult to analyze its stability than a conventional op amp circuit using negative feedback only. However, it is doable. The particular load connected to the current pump output will make a big difference whether the circuit is stable or not.

    You will need to develop an electrical equivalent of the intended load before embarking on the stability subject. I suspect a human body is primarily resistive with some amount of capacitance, but what those are would require more study. Note that TI may not be able to provide assistance with this kind of application because of the human safety implications.

    Regards, Thomas
    Precision Amplifiers Applications Engineering