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Thread: Audio + Return currents (Crossing signals under board)

  1. #1
    Senior Member propa's Avatar
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    Audio + Return currents (Crossing signals under board)

    Hi,

    Long time since I've been here last! My friend criticised my board layouts in the past due to neglecting to take into consideration return currents, stating that ideally you want a clear path underneath the signal, as return current flows underneath the board.

    I've read a few things about path of least resistance and path of least impedance for high speed signals, but I'm unclear whether this would actually affect layouts for Codecs for example. Do the signals for the audio codecs eg MCLK LRCLK, BLCK run at such a speed where a designer really needs to pay attention to return current paths?

    In practice I'm trying to route a board for the CS42448, as I found that the Teensy library supports it. I've read the datasheet, and does mention to leave the Clock signals away from the FLIT+ parts to avoid coupling into the modulators (https://www.mouser.com/datasheet/2/7..._F4-275685.pdf page 39)

    The datasheet also states to leave a copper pour under the IC with vias connected to ground plane on other side of of board. If you were to fill the entire space under the bottom of the IC with a copper pour, then routing signals becomes a bit more complex, as LRCLK and BCLK need to be connected to two sides of the chip, it's easier to route if you connect to those two under the chip.

    So I guess my question or invitation for advice would be along the lines of; there's no avoiding signals will eventually cross, especially on smaller boards, but how much and when does it matter for audio? Should I just be concerned with return current on the main power signals, or every single signal? Does it really matter about bottom traces running perpendicular to another signal on the top?

    It's just a bit of criticism, but it's really hindered my ability to finish anything off now, as I'm 2nd guessing every trace and decision. I'd really like to get some reassurance or people's personal takes/intuition/advice etc

    Cheers in advance and apologies if this isn't directly Teensy related, but there's an amazing wealth of knowledge and you've all been really great at disseminating information, so I hope it's not too out of place. TIA!

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    Senior Member propa's Avatar
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    Quote Originally Posted by MarkT View Post
    Excellent article, confirmed a lot of things for me, it's 2am so need to have a rest. Will jump on it tomorrow and see if I can put these tips to good use! Thank you!

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    Senior Member propa's Avatar
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    @MarkT Thanks so much for the article, I've reckon I've just got to the first pass of codec work...Click image for larger version. 

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    3v3 crosses SDA, and 5v wraps around the board. 3v3 comes out of the Teensy 3v pin in two directions, one feeding one half of the 3v3 circuit and the other feeding the other half.

    When you're on a two layer board and routing a bunch of power signals to the caps, does each power section need a path to 3v3, or can they be tied together in one big net? For instance in mine there's 3v3 decoupling caps in top right of the IC, and one of the ends of the caps also goes to 3v3 of the reset chip. Is that bad practice? Should I make a wind like the 5v round the outside and feed each 3v3 separately? Is there any difference from taking the power net from the de-coupling caps and feeding it to another chip?

    Sorry if this is a dumb question. I'm just extra worried as I've never tried anything this complex before. Routed opamp stuff, but managed to get away with my previous careless and sloppy routing! I think these digital chips won't play nicely if I'm not paying attention!

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    Senior Member propa's Avatar
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    Click image for larger version. 

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    Click image for larger version. 

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    OK so I had a final go at the codec board. Not sure it's perfect, in an attempt to save space I've had to move some of the 3v3 decoupling caps away from the IC, which probably isn't the greatest idea.

    Any ideas to save more space? Or should I instead make it a little wider? It's four layers, and 3v3 and 5v are on the inside Route2 and Route15. First time doing a 4layer, anyone with experience doing mulitlayer have any advice or see anything immediately wrong?

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