Return Path Capacitors

Discussion in 'General Electronics Chat' started by nickagian, Feb 3, 2015.

  1. nickagian

    Thread Starter Active Member

    Mar 12, 2010
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    Hi everyone!
    In the schematics of a reference design of an Ethernet Switch with 10G Interfaces, I have seen that the designer has used several capacitors that he called "Return Path Capacitors". These are 100nF and placed between the power supplies (see attachment). Some also placed towards ground.

    Does anybody know what's the purpose of these capacitors? If yes, where should they be placed physically on the PCB? How many to use?

    Thanks in advance!
    Nikos
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Which company is that design from?
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    There is a post for this:

    http://forum.allaboutcircuits.com/threads/decoupling-or-bypass-capacitors-why.45583/

    Decoupling capacitors are used to remove high frequency noise on the power supply lines caused by switching circuits found in every IC.
    Capacitors must have short leads (SMD are best) and must be located at the IC as close as physically possible between Vcc power and GND with no additional leads or PCB traces.
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You might notice that these are between different power supply levels, not to ground.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    My guess is that these are stitching capacitors.

    Say you have high speed signals on the top of a four layer PCB and on the layer just below that are your power supply planes. The signals will produce return image signals in the power planes that will try to be right under the top layer signals. But when you cross power planes the return image signal has to deviate until it can find a path around the discontinuity. The results in a current loop that acts as both a transmitting and a receiving antenna for EMI and can cause EMC problems. So you place a small capacitor between the planes as physically close to where the signals cross as you can to give the return image signal a small loop to work with.

    What you are doing is turning all of your supply planes into one big multi-point ground plane for high frequency signals while keeping them separate and probably organized along the lines of a single-point ground for DC and low frequencies.
     
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  6. nickagian

    Thread Starter Active Member

    Mar 12, 2010
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    Thanks a lot for the Explanation!! That really made it clear to me.

    I still haven't got sth though. So you mean that I have to locate where the return paths or where the high speed signals cross the discontinuity between two different power supply planes and place these caps somewhere there? I suppose you mean sth like this, right?
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    What is 'sth'?

    The image currents will try to be as close to the signal currents as they can be. That's where you want them, so you use the stitching capacitors in order to provide that path and to keep it as close to the signal lines as you can.
     
  8. nickagian

    Thread Starter Active Member

    Mar 12, 2010
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    I just ment what I asked: "So you mean that ...", regarding the physical location of the caps. Thanks for your reply!
     
  9. WBahn

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    Mar 31, 2012
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    No problem.

    I still don't get what 'sth' stands for. I think I follow what you meant by it -- I just can't figure out why those three letters. But then, I'm not into text speak (which is what I assume it is). Maybe I'll try to look it up.
     
  10. nickagian

    Thread Starter Active Member

    Mar 12, 2010
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    Sorry, I didn't get what you did't understand. So with 'sth' I ment 'something'... I think this is a a standard abbreviation for something then of course I am not native english speaker!! So I maybe wrong!
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    It may be a "standard" abbereviation, but we try to not use any abrreviations, because it makes communication ambiguious and difficult.
     
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