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LM124: LM124

Richard Cummaro
Prodigy 70 points
Part Number: LM124

Gentlemen:

I recently had to add an RC snubber to the output of a differential amplifier (LM124) which is used as a feedback sense of the output voltage of a high frequency (900Khz) switching Buck regulator. The value of the snubber is 560pf and 10 ohms. I did this (and I don't like adding it!) to alleviate the switching noise that seems to get coupled into the output of the diff-amp. I would like to know what the maximum capacitance is that I can safely use that would not risk instability of the diff-amp (LM124).

BTW, this switching noise, which is directly dependent on the dv/dt of the switching transistor seems to be coupling in via the bias supplies (+12V, -0.7V), and not the inputs. Beefing up the bypass caps on the supplies helped but did not eliminate the noise coupling.

Any insight into this phenomenon would be greatly appreciated.

Thank you,

Richard Cummaro

Schematic:

LM124 for output voltage sense

  • 0
    TI__Genius 13660 points

    Hi Richard, 

    Is this related to your previous thread you posted? https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1357068/lm124-lm124-quad-op-amp 

    If so, please hit "Reply" next time when you respond to the thread. 

    Could you show what the output stage is connected to? And what the resistor values are as the image is blurry so cannot see the resistor values clearly.

    Thank you!
    Best Regards,
    Ashley

  • 0 in reply to Richard Cummaro
    TI__Genius 13660 points

    Hi Richard, 

    I'm not quite sure if you updated something, but I do not see additional information added in your thread. Please correct me if I am wrong. 

    Regards,
    Ashley

  • 0
    Prodigy 70 points

    Ashley,

    I'm sorry the figures are blurry. I make clean copies with my snipping tool, but when I paste in this block they become blurry. 

    Essentially, there is a differential amp (LM124) feeding back the output voltage. Gain of 0.1466. Resistors are 3.65k and 24.9k.

    The output goes through another divider (10k/10K) and then to the inverting input of the feedback amplifier in the 1823 pwm chip as shown above.

    Hope that helps your understanding of how I'm using the LM124.

    Thanks,

    Richard

  • 0 in reply to Richard Cummaro
    Guru 298340 points

    To insert images without rescaling, remove the "Dimensions" numbers.

  • 0 in reply to Clemens Ladisch
    TI__Genius 13660 points

    Hi Richard, 

    Thank you for the additional images. To clarify my question, I am not sure I understand how the output and IN+ is connected on from the op amp side to the second image. Also what are the supply voltage values? 

     

    Is the output of the op amp connected here circled in red? 

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 70 points

    Yes, the output of the op amp is connected where you have circled.

    The supply voltages are +12V and -0.7V. 

    Also, that RC on the op amp output is 10 ohms and 560pf. It was put there to smooth out some high frequency noise caused by the switching power mosfet. Don't like to have it there (and it shouldn't be there!), but I can't filter out the switching noise that is getting in with any other means. Is the 560pf too big? Could it cause instability to the op amp?

    Thx,

    Richard

  • 0 in reply to Ashley Kang
    Prodigy 70 points

    Also, there are bypass capacitors on each supply (0.1uf)

  • 0 in reply to Richard Cummaro
    TI__Genius 13660 points

    Hi Richard, 

    Is there a reason you have the op amp connected in positive feedback? The op amp was initially set up as a differential circuit which should not require positive feedback. Unless the C2003 (560pf) and R196 (3.65k) is actually tied to ground? 

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 70 points

    No Positive feedback. It's configured as a differential amplifier. Yes, the RC is tied to logic return. Put there only for filtering purposes as I mentioned above.

    Richard

  • 0 in reply to Richard Cummaro
    TI__Genius 13660 points

    Hi Richard, 

    Thank you for the additional information. Is it possible to put a filter on the input side instead? With 900kHz switching frequency, you may also be getting CMRR issues with the amplifier. Could you provide an oscilloscope capture of the inputs (both IN+ and IN-) and output without the snubber? 

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 70 points

    Hi Ashley,

    I tried putting caps on the inputs but it did not work. Looking at each input and the output, we see that the positive input is clean and the negative input has the ripple which is fed back from the output. I'll see if I can find some scope pictures. Noise seems to be coupling into the output. Maybe a case of the layout not being optimal. 

    Thanks,

    Richard

  • 0 in reply to Richard Cummaro
    TI__Genius 13660 points

    Hi Richard, 

    Yes, please provide the scope pictures with and without the snubber. It would be helpful to see how the output changes from that. 

    Thank you!
    Regards,
    Ashley