[SI-LIST] Re: Plane breaks - Presentation download

My comments were specifically directed towards the split plane crossing 
aspects of high speed single ended nets, such as those used in high 
performane memory systems.  With the large numbers of power supplies 
required by current generation ASICs and FPGAs, it is not uncommon for a 
designer to have to make a decision about whether to route some signals 
across a power island split.  And, even if they don't make this 
decision, they may inadvertently cross a natural split as signals change 
from a PCB plane reference to a Memory Module plane reference through a 
connector.  Passing high speed single ended signals through either of 
these types of plane splits can be disasterous. However, what is often 
neglected are the serious issues with passing differential pairs across 
splits.
Unfortunately, differential pair nets rarely carry "just" the 
differential mode.  But, most who have analyzed the problem have 
analyzed only the differential mode. In fact, significant common mode 
energy exists on differential pair nets from the following sources:

    Driver launch skew.
    Package skew.
    Package crosstalk.
    Package PDN noise
    PCB PDN noise
    Trace routing skew
    Laminate weave induced skew  (come to our paper on this at DesignCon
    2005)

It is not uncommon to find that differential matching skew has been 
degraded to as much as 10 ps/in.  The common mode conversion that this 
introduced by these skews can lead to some extremely high values of 
crosstalk across a split.  Yet, differential signals generally have much 
less immunity to common mode noise than single ended signals. 

Personally, I'd think twice before passing even a differential pair 
across a split plane.


scott

-- 
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed is the registered service mark of 
Teraspeed Consulting Group LLC



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