I'm planning on getting Audio Data from a MEMS Analog Microphone with an OPA344Nna as the OP-AMP.

I'd like to just make sure the audio circuit looks correct. (Analog/Audio is something a bit new to me, but I'd like to get it working for wake words / other)

I'm planning on taking readings using the ESP32's built-in I2S Analog read with 16Khz and 2 buffers for 1-second audio data (further processing done on this later, but hardware first!). What I'm asking is if the circuit looks correct.
Any advice or suggestions is welcome!
The whole application is a smartwatch, please refer to the sensors section for the audio schematic.

enter image description here

  • 1
    \$\begingroup\$ What's the R6 for? With those resistor values, it makes no sense and messes up the bias. The gain is also 1000x or +60dB, is so much gain really necessary? \$\endgroup\$
    – Justme
    Commented Jul 22, 2021 at 19:21
  • \$\begingroup\$ In addition to what @Justme said, what's your bandwidth requirement? With a gain of 60dB you're somewhere around 1kHz for the OPA344. Maybe that's OK for your application, but with no spec we don't know. \$\endgroup\$
    – John D
    Commented Jul 22, 2021 at 19:31
  • \$\begingroup\$ I copied it from a sparkfun schematic using the same OP-AMP, but looking through the data sheets and application circuits I think it is not correct. Thank you! \$\endgroup\$
    – Caleb
    Commented Jul 22, 2021 at 20:38
  • \$\begingroup\$ I think R26 should be 300K with a 27pF Capacitor (C17) \$\endgroup\$
    – Caleb
    Commented Jul 22, 2021 at 20:54
  • \$\begingroup\$ @Caleb No, the SparkFun schematics are fine, your copy is not correct. And you need to have the RC cutoff below 8 kHz if you are going to sample at 16 kHz, to prevent aliasing. Maybe much below 8 kHz since that's a only a single pole filter. \$\endgroup\$
    – Justme
    Commented Jul 22, 2021 at 21:19

2 Answers 2


enter image description here As shown, for self-bias, crystal load and audio spectrum fixes.

  • \$\begingroup\$ Thanks for your help, I realized R6 should not be there and will change the values accordingly. The External RTC Crystal Resistor was shown in the Espressif Datasheet, should it not be there? \$\endgroup\$
    – Caleb
    Commented Jul 23, 2021 at 2:34
  • \$\begingroup\$ There ought to be a reference to 1M to 10M with an internal option never 1k unless in series \$\endgroup\$ Commented Jul 23, 2021 at 2:38
  • \$\begingroup\$ Oh I see, So change the value to 1-10M, or leave it off? \$\endgroup\$
    – Caleb
    Commented Jul 23, 2021 at 2:43

There is no datasheet available for the old NSM2402AT mems mic. The opamp will not work with R6 caising its output to be at GND. The OPA344 opamp is too noisy (hiss) to be a mic preamp. The opamp gain of 1000 is way too high which cuts 1kHz and all higher audio frequencies. mic preamp


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.