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PDM convert to analog

Dear Sir or Madam,

in my studies I am working on a project to measure 2 microphones.

For this purpose a small sound absorber chamber is built to measure with
measurement electronics an analog reference microphone (the b&k 4165) and a DUT
(e.g. the digital microphone Infineon IM69D130-DS).

We used the 4165 mic as well as a suitable preamp and the FFT
Analyzer 3160 from the company B&K.

The FFT Analyzer requires analog signals at the input and converts them to
digital signals with a sampling rate of 131kHz.

My problem is to pick up the digital microphone IM69D130-DS. This
MEMS micro gives as signal a 1-bit PDM signal, which I have to convert to an
analog signal, so that the FFT Analyzer recognizes it.

Do you offer such an interface or chip that turns a PDM signal into an analog one?

kind regards



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[edited by: Ovid at 2:05 PM (GMT -4) on 4 Jun 2022]
  • Hi Ovid,

    We cannot support products from other manufacturers (like Infineon).

    But one suggestion - you should find a way to capture the digital data from the microphone directly. If you convert to analog, then back to digital (in the FFT analyzer), you've got two additional conversions in the analog domain, both of which will add their own noise and distortion.

    The ADAU7002 may be useful - I am not familiar with that part myself but it looks like it simply converts PDM to I2S, which may be much easier to capture. You can post questions about this part in the Audio EZ forum.

    Also, use your favorite search engine to search "i2s audio analyzer" - that may turn up some ideas.

    -Mark

  • Yes I know, the point is just that, the FFT Analyzer is part of the measurement equipment what I am supposed to use. So I can't get around making an analog signal out of the digital one. Unless there is a possibility to feed the digital signal into the analyzer behind the integrated A/D converter.
    Then I save this conversion and with it also losses and disturbances.

    However, I still don't know what kind of digital signal the analyzer uses. I can't find anything about it in the data sheets, let's see what the manufacturer says about it.


    What exactly is I2S supposed to be for a signal? Why just that?

    But thanks for the quick help :)

    best regards

  • Indeed, if the FFT analyzer is targeted at audio applications, it may have a digital input. The Stanford Resarch SR1 has optical / TOSLINK inputs. I've never used them, but there are I2S to TOSLINK converters (do an internet search), and as noted above, PDM to I2S converters.

    You will have to do some research and experimentation to see what will actually work, based on what your analyzer will accept. If it is ONLY analog, then yet another option would be to use PDM to I2S, capture the I2S data as an audio file (WAV or some compressed, lossless format), then finally, play back the file through a high-quality sound card to your analyzer. Compare the sound card specs to the expected microphone performance, to make sure that the sound card is NOT the limiting factor.

    Again, we can only provide support for Analog Devices products, and if you have specific questions about the ADAU7002 (assuming that's useful to you), check the Engineer Zone Audio forum.

    Good luck!

    -Mark

  • I think the closest thing to a single PDM to "analog" output chip we offer is one or another of these kind of PDM to class D audio amplifiers from our Maxim parts:

    https://datasheets.maximintegrated.com/en/ds/MAX98358.pdf

    The output will be way bigger / more power than you need to send to the analyzer and will need to be filtered to remove the class D clock switching but with little or now output load and heavy anti-aliasing filter it might have low enough distortion.

    There are others in the catalog so do a search to see them all and check the data sheets to see it they might be a fit for your needs.

  • Thanks guys for the helpful info, I'll take a look.
    And if I want to get help on the design then I'll turn to the "Audio" subforum, thanks for the tip!
    best regards