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PCB design

Hi all,


I'm looking for some help with PCB design.  Specifically, I'd like to collect data from a large number of MEMS microphones mounted on a single board.  The mics are PDM devices that are clocked at about 3mHz, and then downsampled to 16 bit samples at 48kHz.  At 3mHz, I'm concerned that the wires will act like antennas, and interfere with each other.  Can anyone advise on best practice (or provide a reference)?


Cheers,

 

Carey
  • I was reading that as millihertz. You mean 3MHz? Good ground planes, and a signal return path paired to the outgoing path, reduce undesirable effects. Are you talking dozens of microphones?
  • Thanks for your response, Alex.  Yes I should have written MHz (3e6).  To answer your question, this project will use roughly 100 microphones.  Do you have experience with MEMS mics?


  • MEMS accelerometers yes, microphones no. Are they SPI output, given the 3MHz clock?
  • No, they are PDM (Pulse Density Modulation) outputs that are basically a single bit per clock cycle.  The output is then integrated and subsampled to produce a 16 bit PCM signal at 48kHz. 


    What projects have you worked on with accelerometers?

  • Mostly sports measurement, and mostly analogue output, for other sensors mostly SPI. Is the clock shared across all your devices or one per device?
  • That's a good question... Although I have an  EE degree, until recently I never thought I would be designing a PCB, but all of a sudden I have a project, and I'm trying to pick it up as fast as I can :)  Perhapst there are learning resources that you could recommend?


    I was thinking one clock would be shared by all the mics, but perhaps there are restrictions on what can be achieved with a single clock.


    It would be great to have a longer chat if you have time.  Are you in London?
  • No, I'm afraid not, I ride my tractor around Norfolk. 3MHz, 100 devices, needs a little bit of thought.
  • Cheers, Alex :)
  • I've recently done some signal distribution across 100 devices, but converted to differential pairs for transmission, as this is much more benign than a single-ended signal flapping about. What are your distances between devices & source?
  • The acoustic sources are in the farfield (> 20 wavelengths from the board).  The microphones themselves are regularly distributed over a surface of about 80 cm in diameter.  This means that some mics could be as much as 40 cm away from their signal destination on the board.  I guess the SNR of the 1-bit signals on each wire will be affected by distance, but don't know by how much.  Just wondering if there is a circuit modeling program that helps mitigate issues.  I'm looking into Autodesk Eagle for features like that.