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Unexpected behavior observed on OPA4170 based circuit

Prodigy 80 points

Replies: 5

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Hi,

We are using OPA4170 based control circuit in one of the design. When probed on the pins of OPA4170, I am observing unexpected voltages.

The schematic is attached herewith. With the input to the U22.12 through voltage divider circuit as 0V, I am observing the following voltages.

U22.1 = 1.27V

U22.2 = 0.448V

U22.3 = 0.406V

U22.5 = 1.393V

U22.6 = 0.216V

U22.7 = 11.97V

U22.12 = 2.4V

U22.13 = 3.5V

U22.14 = 0V

Can you please identify where is the issue?

Thanks,

Dixit

Analysis-TI.pdf

5 Replies

  • Hi Dixit,

    I do find it unusual that when you apply 0 V to the top of the voltage divider connected to OPA4170, pin 12, and you measure +2.4 V at the divider center point . I would expect that point to measure 0 V there as well. 

    Overall, the circuit has two integrators (sections having pins 5 -7, and 12 -14), whose output can pull up against an output supply rails if the circuit is in static, dc mode. That is indicated by the pin 7 integrator output at 11.97 V, and the other integrator output, pin 14, at 0 V. Once an op amp has its output slammed against the rails and is in saturation it no longer acts as an op amp and abnormal voltages can occur at the other pins. I suspect that is why you are measuring odd voltages at the integrator input pins. It would seem you would want the circuit to be active and observe the pins with an oscilloscope. 

    The upper OPA4170 amplifier, the one having the D7 diodes across the inputs, should measure close to 0 V at the diode common point. Measure the output, before R118, and make sure there isn't oscillation. 

    Regards, Thomas

    PA - Linear Applications Engineering

  • In reply to Thomas Kuehl:

    Hi Thomas,

    Thanks for the reply and Sorry for the late reply as my internet connection was bad.

    I had probed the pins of the op-amp with oscilloscope and found that certain pins of the op-amp exhibited oscillations. I have attached the screenshot of the waveform obtained at U22.5 with respect to 24V_FILT_RTN. The same noise is appearing across 24V_FILT_RTN and GND (screenshot shown below) eventhough we are shorting between the GNDs at the isolated converter(breaking the isolation). Do you think we need to do this near to U22 IC also to avoid the noise?

    I believe the noise is causing the unexpected behavior of the circuit. Is my understanding correct/

    Thanks,

    Dixit

  • In reply to Dixit Jones Zachariah:

    Hi Dixit,

    The waveform that you measured looks to be periodic and more transient-like in characteristics than the more sine-like waveform associated with op amp oscillation. Is the circuit powered by linear supplies, or by an on-board buck/boost switching regulator?  Is there something else on the board that is switching at ~3 us rate?  

    If you haven't, have you considered setting your circuit up in a circuit simulator? It's not a terribly complex circuit and doing so would allow you to observe the various circuit nodes as they get exercised. If you don't have a simulator you can get TINA-TI for free, from our website. It is a quick learn, intuitive and easy to use.

    Regards, Thomas

    PA - Linear Applications Engineering

  • In reply to Thomas Kuehl:

    Hi Thomas,

    We found that the noise found on the op-amp pin as from the isolated converter used on the board which is switching at frequency 375kHz. The noise is identified to be common mode noise from the converter. There was a mistake in my previous mail which had shown the noise from U22.5 to GND actually and not U22.5 to 24V_FILT_RTN. We found that having the capacitor, C90 to GND instead of 24V_FILT_RTN solves the problem and my op-amp is behaving as expected.

    More workaround has to be done further on to retain the C90 with 24V_FILT_RTN and to have binding capacitors between the GND and 24V_FILT_RTN near the U22 IC.

    Thanks for the help and I will update you further.

    Regards,
    Dixit
  • In reply to Dixit Jones Zachariah:

    Hi Dixit,

    Wow, those are some good finds! I am glad to hear your circuit is now functioning as expected.

    If you need anything, we are here to help.

    Regards, Thomas

    PA - Linear Applications Engineering

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