[SI-LIST] Re: reference plane

A few thoughts, extracted from my response to a similar question...

The original question from last October:
An internal signal layer in a multi-layer stackup is situated between ground
and a power plane. If the power plane splits into two islands is the effect
significant on the signal layer?  (Dielectric thickness between layers = 8
mil.)

My response:
Assuming your planes are very wide, and since the space between the planes
is kept small, there shouldn't be any significant effect to Z0 at these
geometries.

I had the opportunity to measure impedances of stripline traces which run
between ground and power planes.  I found that the TDR was identical
regardless of which plane I measured relative to.  After some investigation,
I concluded that for the geometries involved (only 14mils between planes),
the coupling between the planes was strong enough that, AC-wise, they were
one and the same.

The identical situation should hold true for your configuration, if your
planes are wide enough to have strong coupling between them and ground, and
the space between the 2 planes isn't wide enough to cause a discontinuity.



In summary,
If there is strong coupling between the ground and power planes, they are
essentially the same for high-speed reference.  If they aren't strongly
coupled, there will definitely be a difference between which is the
reference.  People have used "stitching caps." (capacitors between the two
planes at the location where the signal transitions from one reference to
another) to remedy the problem, but I'm skeptical of the value of these.
For the rise-times we're at today, I believe those caps. would have little
real value (the parasitics would be too great).  I.E., a TDR done without
the caps. populated would look the same as one done with the caps. in place.

I believe the best approach is to either keep referenced to the same plane
throughout the entire topology (and most chips reference signals to ground),
or only transition between planes which are strongly coupled.

My 2 cents.

Jeff Loyer

-----Original Message-----
From: evillaf@xxxxxxxxx [mailto:evillaf@xxxxxxxxx]
Sent: Thursday, June 20, 2002 10:36 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] reference plane


Can anyone explain the advantages and disadvantages of 
using a power plane for a reference plane instead of a 
ground plane?  I have always used both as a DC reference 
in the past.  Now I am beginning to hear arguments that 
only GND planes should be used for critical signals.  
This becomes somewhat impractical for some boards.

Thanks in advance.
Ellis
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