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I2C noise into the audio signals

Discussion in 'Electronic Design' started by [email protected], Dec 9, 2013.

  1. Guest

    We have TDA7719 audio processor and OLED display connected to the same I2C bus as I2C slaves. Both the devices are driven by MCU which is I2C master. After TDA7719 is initialized with I2C for input selection and volume, it starts working. The MCU keeps refreshing the display by continuously writing data to the display through the same I2C bus. This I2C noise is getting coupled into the audio signals and I can hear the rhythmic noise from the speakers proportionate to the display refreshing. If the display is paused the noise also pauses.

    What could be the issue? Appreciate any help.

    Both MCU and TDA7719 are powered by separate regulators. Input to the TDA is from MCU which has built in DAC. Problem in the audio source from MCU+DACis already ruled out as direct connection to the amplifier bypassing TDA is outputting clean audio output.

    Thanks
    markj
     
  2. RobertMacy

    RobertMacy Guest

    wow, a FUN project!

    Do you know the magnitude of the sound? Are you hearing 60dB down, or
    80dB down? Could be 'GND' noise coupling in. The 'exra' demand of the
    power could be shifting the gnds out from under your audio stuff.

    If you provide an email address I can send a .zip file to, I'll send you a
    sample of the results of a PCB Layout Tool I created to solve those EXACT
    problems. [gmail accounts seem to reject a .zip file and never tell
    anyone] The tool was created during the design of seismic monitoring
    electronics and the gnd noise was kept to less tha 1/4 LSB. Also did a lot
    of work to make ALL the power filters lossy over their WHOLE bandwidth,
    even into the GHz ranges. We're talking about a filter that looks like a
    huge series resistance and a short to gnd. A lot of people don't notice
    that most of the filters 'recommended' by the mfgr actually go into high
    impedance at certain spectral locations. That translates to having
    absolutely NO bypass at those tones. For example, bypass down to 0.1 ohm
    great!, but most of those filters at some frequency suddenly go to 10 and
    even 100 ohms. No problem *if* you have no frequencies there, but a big
    problem if you do! Put a scope probe on your power and you'll see 'ringy'
    waveforms, that's the effect.

    Another place to look is magnetic fields coupling across traces. MagFields
    are INSIDIOUS! Your scope will barely show them, yet they can induce
    audible stuff everywhere.

    then there's the AM modulated HF that gets rectified somewhere, too.

    I'm probably preaching to the choir on all this. Right now, I'd go forthe
    GND shifting.
     
  3. Guest

    No John. Output of TDA is fed to TPA3110D2 amplifier.
     
  4. Isn't the obvious path via the I2C wires? I was wondering if the I2C speed could be slowed down (for the audio IC).. and then add some RC filters on the I2C input wires. Then see if that reduced the interference. (or is that a silly idea?)
    I guess the first job is to figure out how it's getting in.

    George H.
     
  5. Guest

    You can send the zip to markjsunil at live dot com

    The power supply should be OK as LM317L SOT89 pack can supply 100ma and TDA7719 max current is 35ma.
     
  6. RobertMacy

    RobertMacy Guest

    sent, look forward to your comments

    not to be too ascerbic, but what does that PS chip have to do with
    anything? From memory that chip's not so good either.

    the ratings you quoted are DC, check the spec sheet and you'll see a
    fairly high impedance even as low as 10KHz. That's why caps are usually
    added. BUT! a lot of these 3 terminal regulators go through a huge output
    impedance spike for your whole PS system depending on the value of cap you
    use. TI makes a great series of 3T's that let you include the cap in the
    feedback loop minimizing that effect.

    John mentioned slowing the I2C edges. That can solve a lot of problems
    even with the source coming from different reasons. ie, capacitive feed,
    gnd spikes, ps spikes, magnetic feed, etc etc.

    Be careful shorting energy to ground WITHOUT providing a way to starve the
    energy supply, else the problem can simply exacerbate itself. From years
    of working in EMC mitigation learned the hard way that bypassing doesn't
    always do what you think it does. *IF* you don't include some series
    resistance. Just have to change your thinking from voltage in time, to
    ENERGY in time, then you can find where to stomp on unwanted signals.
    After all, at those higher frequencies, nature doesn't care if it's
    voltage OR current flying around.
     
  7. Short circuit on i2c lines? MCU driving it active hi?
     
  8. Guest

    Right. Short the input and see if the noise is still there. Then use
    an external supply for the TDA. Then...
     
  9. Guest

    shorted the inputs and noise is still there. It already has separate regulator but the power source is same for both MCU regulator and TDA regulator. I will try with 2 separate power sources.
     
  10. Guest

    Can you disconnect I2C from the device?
     
  11. Joerg

    Joerg Guest

    Nah, let's call it slightly suboptimal :)

    Seconded. That is most likely the only way to make the noise go away in
    this case.

    In the audio spectrum it will not help because you can't round them this
    much.
     
  12. Guest

    My first reaction is to look for common paths (PCB tracks or wires)
    for both audio ground and digital ground.

    Some digital noise in the 300-6000 Hz frequency range could be easily
    heard 60-80 dB down from the analog audio signal. Thus, any digital
    signal that causes a microvolt voltage drop in the common ground paths
    can be audible.
     
  13. Guest

    One way to get rid of any digital intrusions into analog signals is to
    make sure that the digital signal does not contain loooong sequences
    of "1" or "0", which could create frequency components below 20 kHz.
    Manchester coding etc. could help a lot.
     
  14. OK then some other path. So I'd try and figure out what the path might be.There's common ground stuff, capacitive coupling and then inductivce effects because of 'bad' ground returns. (and maybe other means as well.) The inductive and capacitive stuff will depend on the edges. Whereas the common ground can be like a resistive addition... So if you slow down or speed up the I2C edges and it gets better (or worse)* that tells you something. If it's roughly independent of the edges then perhaps look for common groundreturn issues. Can you see the pickup on your 'scope? Posting a 'scope shot might help.

    George H.
    *in trouble shooting I often find it easier to make it worse.. before I canmake it better. :^)
     
  15. Joerg

    Joerg Guest

    With I2C you don't have much of a choice, the timing windows aren't like
    with SPI. Also, the human ear is super sensitive to noises that aren't
    smooth. Even a tiny pulse at low duty cycle but repeated within the
    audio spectrum can be heard.
     
  16. Guest

    In professional high quality audio applications either galvanic
    isolation (audio transformer) or at least differential signaling is
    used to avoid all of the ground resistance and most of the
    inductive/capacitice problems on connections between equipments in
    separate boxes.

    Maybe the OP should look at the situation from this point of view even
    for communication between PCBs.
     
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