[PCB_FORUM] Re: Power plane -vs- gnd path signal return path

Hi Jim

A really good question.  I believe your assumption is basically right.

You could argue that single ended signals are usually CMOS, so it doesn't 
matter if they reference ground or power planes.    You would be best to 
closely couple the 3V3 to a ground plane to help cover both the rising and the 
falling edge.

If the power plane is not the same voltage as the drivers, then try get the 
ground plane next to the signal.  If this is not possible, close coupling 
between the power plane and the ground plane will help.

Differential pairs may be different because they tend to be terminated high.  
(Usually PECL.)  Therefore you could say that it's better referencing then to 
the power plane what the driver is powered by.  In reality?  It probably 
doesn't make too much difference.

Discontinuity can be the killer.  If you have to "jump" layers, use a 
transition via to join the reference planes together close to where the signal 
via is.  Then there will be no (or not as much) discontinuity.  This can be 
done with diff pairs as well, but you have to be more careful - try keep it all 
symmetrical.

Hope this helps.  Also, William and Alan had good points.

Cheers,
Richard

  
__________________________
Richard Moffat
PCB CAD Team Leader
Allied Telesis Labs
ph. +64 (3) 3393000
richard.moffat@xxxxxxxxxxxxxxxxxxx

>>> "Jim Wages" <jwages@xxxxxxxxx> 14/03/2008 2:20 a.m. >>>
Ok, folks. Let me know if my thinking is wrong here.

I have a 14 layer stack up. External etch layers are adjacent to power
planes, which are adjacent to ground planes. Lyr 2 is a split power plane.
Lyr13 is a solid 3.3V plane.
Do you see any signal "return path" or integrity issues with routing a
critical signal trace on the bottom layer as long as the driver of the
signal was powered by 3.3V. What about a differential pair?
I'm assuming that the power and gnd planes would be perceived as a short to
hi-frequency signals, so that the return path would not have to get to a
ground plane, but would just stay on the 3.3V plane directly beneath the
trace until it returned to the driver.
Thoughts?


Jim S. Wages
SR. PCB Layout Designer


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