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OPA4172: Audio signal bleeding into other channels (QUAD microphone preamp)

Prodigy 110 points

Replies: 15

Views: 1479

Part Number: OPA4172

Hey

I've created my first PCB design 
using the design from http://www.ti.com/tool/TIPD181 as a basisfor the quad channel preamp OPA4172.
circuit wire up:
https://imgur.com/1e0fYMw

Pcb Design without ground plane showing:
https://imgur.com/DmMtIYb


Pcb with ground plane:
https://imgur.com/5kgW2RJ 

My issue is that all my inputs (mic 1-4) are all showing up across all outputs. Eg; mic 1 will show at all outputs, 2 at all outputs etc etc.
I've tried making another circuit with no ground plane in the case that the PCB tracks were crossing with the outputs but it sounds awful and still has the same issue.

Thanks

  • Edit: Audio feed is perfectly fine, just the signal is appearing at each output
  • In reply to Andreas Metaxas:

    Hi Andreas,

    how much layers does your board have?

    What microphones do you use?

    All the decoupling caps must be located much closer to the OPAmp. Otherwise the OPAmp can become unstable.

    Kai
  • In reply to kai klaas69:

    Hey , this one has 4 layers, one being a dedicated ground layer.
    the recording audio was suitable for what im doing, could the decoupling caps really cause such an issue ?
  • In reply to Andreas Metaxas:

    Hi Andreas,

    please don't feel offended, but the layout is highly disadvantageous. :-)

    You should move the components much closer to the OPAmp. This is especially true for the decoupling caps and filtering caps. You should avoid long copper traces and minimize loop areas. And last but not least, you must strictly separate input and output lines.

    If you use microphones with cabling, take shielded cables and connect the cable shield to signal ground.

    Again, what microphones are you using?

    Kai

  • In reply to kai klaas69:

    Hi Andreas,

    I have a bit analyzed the circuit of the www.ti.com/.../TIPD181.

    It seems that this circuit does not allow any cabling at the input because the microphone line directly connects to the -input of OPAmp. Any cable capacitance would erode the phase margin and destabilize the OPAmp.

    I think a much better approach would be to directly connect each microphone to its individual OPAmp circuit and to avoid any cabling at all. Just like it can be seen in the link. So, it seems to be better to build four individual amplifiers.

    Kai
  • In reply to kai klaas69:

    Fair enough i was hoping that there would be another reason on this weird crossfeeding.
    I've got 4 individual ones ,although through-hole variant so that should cover me till i order the soic ones in.

    Thanks for taking the time to answer my query !

  • In reply to kai klaas69:

    Hey Sorry i didnt answer your prior question

    Im using a cmi-5247tf-k
    dont pay attention to that 1uf cap as thats was a mistake


  • In reply to Andreas Metaxas:

    Hi Andreas,

    Here are a few blogs I've written on PCB layout of an op amp to help in the next revision of your PCB.

    The Basics: How to layout a PCB for an op amp

    How to layout a PCB for high-performance, low-side current sensing designs

    Kai is correct in that long cabling can effect the stability of the amplifier so I recommend either connecting the microphone to the PCB or changing the feedback capacitor to ensure the amplifier is stable with the cable capacitance included in your design.

    Thank you,

    Tim Claycomb

  • In reply to Tim Claycomb:

    appreciate it !
    il check it out
  • In reply to Tim Claycomb:

    Sorry, i had just checked my image that i had attached at the start of the post and had noticed i attached the wrong image.

    For some reason my feedback resistor should be 12p, along with r4 and c6 being 50 olm and 2.2uf respectfully.


    Either way i appreciate the assistance, i resolved the issue using 4 separate boards until i can produce an ideal board

    Regards
    Andreas

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