[SI-LIST] AW: Differential Traces and GND planes

  • From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
  • To: "krishna@xxxxxxxxxxxxxxxxxxx" <krishna@xxxxxxxxxxxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 16 Mar 2012 12:17:20 +0000

In a perfect symmetrical world, differential traces don't need a GND-Plane, but 
the world isn't perfectly symmetric. And the chips are also not pure 
differential, they have common modes as well, and that requires a common mode 
(or single ended) impedance matching.
The material is inhomogenious, the traces each have different etching surface, 
there are pressing tolerances, copper tolerances... And then there are bends 
and local length differences that will drive common modes.

At low speeds, you might get away without a GND plane, below 200ps risetime I 
wouldn't try it.

BR
Gert


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-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im 
Auftrag von krishna@xxxxxxxxxxxxxxxxxxx
Gesendet: Freitag, 16. März 2012 12:47
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Differential Traces and GND planes

Hi Everyone,


Differential traces perfectly cancel out the noise induced on the trace 
provided the length of the two lines are same and are closely tied up all along 
with the controlled impedance maintained throughout the routing.



But what is not sure is



1.      Whether a solid ground plane is needed for a differential pair for
getting the full benefit of differential traces?
2.      Are there any special considerations in making the choice - to
consider providing solid GND plane or get away with it - for say, better 
EMI/EMC and crosstalk performance?
3.      Is there any trade off involved?



Would you please help me get things right here if my understanding is wrong?



Thank you very much,



With Thanks and Best Regards,

Krishna.





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  • » [SI-LIST] AW: Differential Traces and GND planes - Havermann, Gert