[SI-LIST] Re: Split Plane Routing
- From: James_R_Jones@xxxxxxxx
- To: psharkey@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 1 Feb 2002 09:05:05 -0600
Pat,
If you are required to split your ground plane, as you might in a mixed
signal design, then be very careful to keep a return current path with
minimum inductance. You can do this by placing ground vias at the ground
plane ovelaps as close as possible to the traces that are changing layers.
Be advised that you will pay a crosstalk penalty for this, as some
conductors on your bus may have to share return current vias.
If your layer 6 routing is coupled to your power plane (thin dielectric
material relative to ground plane dielectric thickness), you will have to
provide a return current path by placing caps to ground near the layer
trasistion. Here you will suffer crosstalk penalties, and your return
current path will have an increase in inductance associated with the ESL of
the cap.
If there is no way around it, I have seen high speed digital busses work
using these tricks. But my best advice is to avoid the described layer
transitions when possible.
James J.
-----Original Message-----
From: Pat Sharkey [mailto:psharkey@xxxxxxxxxxx]
Sent: Friday, February 01, 2002 8:10 AM
To: Si-List@Freelists. Org
Subject: [SI-LIST] Split Plane Routing
Hello all,
Given the following stackup with planes split in different areas -
1 - S1
2 - GND (SPLIT 1)
3 - S2
4 - GND(SPLIT 2)
5 - POWER(SPLIT 3)
6 - S3
7 - GND (SPLIT 4)
8 - S4.
For a signal which originates on layer 8, should the signal only be
routed on layers 6 & 8? Or, can layers 1 and 3 also be used to cross
a plane split on layer 7, assuming layers 2 and 4 are not split in the
same place (z axis)?
Thanks in advance,
Pat
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