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

  • From: <Wolfgang.Maichen@xxxxxxxxxxxx>
  • To: <krishna@xxxxxxxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 16 Mar 2012 12:07:11 +0000

Without a ground plane you will not be able to achieve 50 Ohm impedance with an 
edge coupled diff pair. Even with a spacing (air gap) equal to line width, and 
assuming 50 Ohm line impedance, ~90% of the return current still flows in the 
ground plane. Removing ground plane here will result in a very high 
characteristic impedance.

Examples without a ground plane would be twisted pair cables, but here the 
spacing is much smaller than the wire diameter.

Wolfgang



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of krishna@xxxxxxxxxxxxxxxxxxx
Sent: Friday, March 16, 2012 12:47 PM
To: si-list@xxxxxxxxxxxxx
Subject: [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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