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ADAU1701 - BCLK & LRCLK Distribution

Hello all,

Following my last post:

I assembled this setup using the EVBs of the ADAU1701 and the external codecs.
I wanted to check if this configuration works even without the use of fanout buffers:
Actually, the series terminations were also not assembled. I used star connections for the BCLK & LRCLK, with different wires lengths.
Even with this simplest implementation, the setup works well (setup functionality only, not signal integrity wise...) 
For the final circuit, I do intend to add serial terminations, and to have control impedance and matched traces lengths for all of the I2S buses.
My question is:
Where can I find information regarding the:
1- Maximum allowed C-Load of the BCLK, LRCLK as output ports? 
2- Maximum input capacitance of BCLK, LRCLK as input ports? (As mentioned, the ADAU1701 is also clocking its own inputs)
Other than the setup is working well,I have no way of knowing if I'm overloading the BCLK, LRCLK lines.
Thanks for helping,
Best Regards,

[edited by: Nir76 at 11:14 AM (GMT 0) on 2 May 2020]
  • Hello,

     First of all, I know it is not a good habit to recover a resolved topic from 2 years ago, but I have almost the same problem as . My design has a ADAU1701 and I want to drive two external audio ADCs. I was searching and found this topic. I would like to know if the resistances at the star connections are needed - and, if so, what are the recommended values.

    I invoque also because of his good answers on these chips topics.

    Thanks in advance. Best regards,


  •   Having just been through this on a design:  For I2S connections, unless the traces are short, it's a good idea to always put in series termination resistors up close to the driving end.  If not for good edges it also may be factor in getting your product to pass EMI testing.  For whatever reason I2S signals seems to end up behind a lot of EMI issues.  The exception would be if the traces are short but there I suggest at least including the series R until you can prove that removing them won't cause problems for either signal integrity nor EMI.

    As to one R per fan out connection, from my experience and reading what other do that seems like the best approach. Though if driving long traces across some very large board buffering would be preferred approach.

  • Hi Brewster,

    Many thanks for your detailed answer, I have the ideas clearer now.



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