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LM4811: Whiff noise on LM4811

Pierre Charon
Prodigy 10 points
Part Number: LM4811

Hello,

I have a bad parasite noise on righ channel with LM4811, which is independent of volume input, but stop if no input signal...it's a background parasite, sound listened is ok.  U/D and clock are connected directly on Atmel microcontroller. Also, I have a parasite blow sound when volume is to maximum.

This is my sketch

Thanks for your help !
  • 0
    TI__Guru** 100475 points

    Hi Pierre,

    Can you please attach some captures of the left and right signals? You mentioned the noise is present only on the right channel, correct?
    Is this noise random or a specific frequency distortion?
    Could you please also test without the speaker connected, basically open the output load connection and check with a scope, is the noise still present?

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Hi Ivan,  I am going to post a wav  file, so you could hear and see...yes on right channel, not random noise, appears when input signal each time...I am afraid to have a bad lot of LM4811, but I have 2 prototypes with this problem, my shematic seems good, really I don't understand what's happens...

    Thanks   Pierre

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Hi,  In fact do you think I should remove C37 and C38 , as I have already C6 and C8 ?

    When I shunt C37 the bad whiff noise seems quite removed !

  • 0 in reply to Pierre Charon
    TI__Guru** 100475 points

    Hi Pierre,

    I think it's a bit odd that it only affects the right channel. Perhaps you can try a couple more things to better understand the root cause:

    • Add a series resistor (some large value in the order of kOhms should work) between C7/C37. Perhaps the internal DC bias (resistor divider) is oscillating in some way passing through C37 and shorted to GND through C7
    • Remove C7 as well, keeping C37. Does this has the same effect?

    Removing C37 itself seems a good workaround, I'll double check this with one of our analog experts and let you know further comments early next week.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Thanks Ivan, yes also on left channel, but lower...

    As the pcb is finished I cannot try to add resistor or remove C7, I can only shunt C37 by connecting a wire handly on the 2 poles of C37, and the result is really better, I 'll try to shunt both left channel, anyway I don't understand why C37 gives this noise...

    Have a nice week end :-)

    Pierre

  • 0 in reply to Pierre Charon
    TI__Guru** 100475 points

    Hi Pierre,

    I'll discuss this further with some of my colleagues and let you know our thoughts on the possible root cause. However seems OK to keep your workaround.
    Thanks and have a good weekend too.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Hi Yvan,

    Also, I must say I have connected GND of the LM4811 to analog ground AGND, may be I should connect it to digital ground GND, I read it on this forum under other topics about LM4811...may be it's the cause of my problem !

    Pierre

  • 0 in reply to Pierre Charon
    TI__Guru** 100475 points

    Hi Pierre,

    What is the GND reference of the audio source? eg. if you're using a DAC at the input of LM4811, what is it's GND reference, digital or analog?

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Hello Yvan, The ground reference for both DAC AKM4384 and LM4811 is the same analog ground (audio source), the bridge digital ground to analog ground is made near the power supply DC plug at around 12 cm of the 2 ICs

    Hope can help !

    Pierre

  • 0 in reply to Pierre Charon
    TI__Guru** 100475 points

    Hi Pierre,

    Can you also specify the materials of the capacitors on your schematic? Are C37/38 polarized electrolytic or something different? I see different symbols across the schematic.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Hello Ivan,

    C37/38 are tantalium Kemet  C6/C8 electrolitic audio Nichicon

    Also please check your colleague topic: https://e2e.ti.com/support/audio-group/audio/f/audio-forum/318120/lm4811-headphone-amplifier-layout

    I think the idea of common AGND on the 2 ICs could be the problem, may be the LM should be connected on GND and not AGND !

    Pierre

  • 0 in reply to Pierre Charon
    TI__Guru** 100475 points

    Hi Pierre,

    After thorough discussion with my team on this and the other thread you added (thanks) I've got the following comment:

    As you mentioned, this problem may be related to GND connection. Generally with analog amplifiers you should treat the GND as two separate portions: Small signal GND (sometimes also referred to as AGND) and Power GND. Care must be taken so that noise on GND connection from power GND back to the power supply is not coupled into the small signal GND. The GND routing from power GND to the power supply GND connection represents some specific impedance, which interacts with the load impedance and thus can get a small noise representative of the output large signal.
    Small signal GND would consider VIN1, VIN2, Shutdown and Bypass pins. Power GND would be VDD decoupling, GND, VOUT1 and VOUT2.
    The most straightforward way to avoid this noise coupling from output GND to input small signal is to use star GND connection to the supply, this way ensuring each loop is not affected by others returning to GND.
    If using a common GND plane, then component placement can be used to help reduce the noise coupling effect by placing the output and power related GND connections from the amplifier closer to the supply connection, and the DAC and input related GND connections farther away from it, this way the noisy power GND would form a smaller loop closer to the supply, not affecting the input signal.

    Hopefully this helps, let me know if there is any further question.

    Best regards,
    -Ivan Salazar
    Applications Engineer

  • 0 in reply to Ivan Salazar
    Prodigy 10 points

    Hi Ivan,

    Thanks for explanations, I am going to try to modify my pcb in this way, by connecting LM4811 GND to GND ground plane and not AGND plane, and will tell you if th problem is solved.

    Pierre

  • 0 in reply to Pierre Charon
    TI__Guru** 100475 points

    Hi Pierre,

    Make sure to connect the GND planes on a single point, either close to the supply connection or in such a place that avoids noise coupling between the different GND planes.
    Let us know if there is any further questions.

    Best regards,
    -Ivan Salazar
    Applications Engineer