I've designed and received a custom MEMS microphone amplifier PCB, but am having issues with poor high frequency response and very low sensitivity/gain. Basically the microphone stops picking up sounds above ~300-400 hz. These are output using a tone generator on a medium-sized speaker system. Playing loud music results in a very small response, and only at lower frequencies. I have tried 5 of these mic boards and all have the same issue, so it's not just a single faulty board.

The circuit diagram is below. It is based off of and essentially identical to the Sparkfun MEMS board. I have tried one of these Sparkfun boards on the same system, power supply, etc, and it works fine, so the issue is with the boards I've received.

Could anyone provide advice on what might be the most likely issue, which components might be trouble, parts of the circuit to test and diagnose, how to diagnose, or any other general ideas? I have a multimeter and can get access to a Oscope if needed. Thank you.

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  • \$\begingroup\$ Try removing C1. If it perks up, then you're using a 10nF cap there instead of 100pF. You still want the cap there (it rolls off the frequency response above 15kHz), but unless your layout is appalling, for a test you can leave it off. \$\endgroup\$ – TimWescott Feb 9 at 0:28
  • \$\begingroup\$ Note that you may get marginally better performance using the OPA345 -- but if there were any difference at all it would small, not the ginormous difference you're seeing. \$\endgroup\$ – TimWescott Feb 9 at 0:30
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    \$\begingroup\$ Can you add your PCB layout - both top & bottom. We may see something that you are overlooking. \$\endgroup\$ – Dwayne Reid Feb 9 at 0:49
  • \$\begingroup\$ Do you have AUDIO_IN in the place of your scheme, whee it must be OUTPUT. The same with OUTPUT, it must be INPUT. \$\endgroup\$ – Khirgiy Mikhail Feb 9 at 2:43
  • \$\begingroup\$ Perhaps this is an acoustical problem, not an electrical one? How does the sound get to the microphone? Note that the sound opening is at the bottom of the package, necessitating a hole in the PCB. \$\endgroup\$ – sh- Feb 10 at 10:52

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