[SI-LIST] Re: Current Return Vias
- From: "Larry Smith" <LSMITH@xxxxxxxxxx>
- To: <joel@xxxxxxxxxx>, <leeritchey@xxxxxxxxxxxxx>
- Date: Mon, 24 Jul 2006 10:54:27 -0700
Joel - You have it right. If transmission line return current must jump
from one reference (ground) plane to another, it is always beneficial to
provide a via to help that happen. Return current can jump from one
ground plane to another through the inter-plane capacitance but this
will involve reference plane bounce. The amount of reference plane
bounce depends upon the number of signals making the transition and
dielectric thickness between planes. =20
One signal will not cause excessive plane bounce (usually several mV)
but several signals switching at once will make an SSN issue in the via
field. I've seen designs where a couple of dozen single ended signals
forced return current to jump from one plane to another, resulting in
excessive power plane bounce, crosstalk between signals and SSN noise on
quiet lines. The more signals you have, and the further apart the
planes are (more dielectric thickness, less capacitance between planes),
the more important it is to provide return current vias.
As Scott mentioned, a bunch of signals that go through vias and force
return current to jump between reference planes is just like sending
signals across a plane split. It is a discontinuity in the return path
which must be considered. It is also like running a bunch of signals
through a connector without providing sufficient return path. We know
better than to do these things. =20
Differential signals can jump reference planes with minimal
consequences. You might even get away with it with just a few single
ended signals. But watch out if you have enough single ended signals to
create SSN problems. The reference plane bounce issue grows with each
additional signal making the transition. =20
Regards,
Larry Smith
Altera Corporation
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Joel Brown
Sent: Sunday, July 23, 2006 12:26 PM
To: leeritchey@xxxxxxxxxxxxx
Cc: Kenny Frohlich; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Current Return Vias
I am currently working on board that has 12 layers.
Ground planes are layers 2, 5, 8, and 11.=20
If a high speed signal changes from layer 3 to layer 9 then its
reference
planes change from layers 2 &5 to layers 8 &11.
Since the return current flows under the signal trace on the reference
plane
, this return current has jump from from one set of planes to the other
set
at the point signal changes layers through a via. The return current
could
jump through interplane capacitance or through a ground via located next
to
the signal via. So in this situation I don't see why having a ground via
nextto a signal would not be beneficial? Does the interplane capacitance
provide a lower impedance path than the ground via for the return
current to
jump planes? Please enlighten me.
Joel
Lee Ritchey wrote: Kenny, It is not true that you need a "return
current"
vianext to each layer changing signal via. I continue to be amazed that
engineers who are looked upon as SI experts say such things. Imagine you
havea 4 layer PCB, such as the mother board in a PC, where there are
only
twoplanes, one Vdd and one ground, where would such vias connect? There
have
been billions of these made to date that work just fine and have very
fast
signals on them. The return currents you are concerned about find their
way
from plane to plane through the collection of decoupling capacitors and
interplane capacitance that you had to engineer into the power delivery
system in order to make it stable. Focus on this and the return currents
takecare of themselves. EMI is minimized he same way.. [Original
Message]
From: Kenny Frohlich <kenny_frohlich@xxxxxxxxx>[1] To:
<si-list@xxxxxxxxxxxxx>[2]Date: 7/22/2006 6:45:56 AM Subject: [SI-LIST]
Current Return Vias Dear Experts, I understand that I need to provide
ground
vias next to via explictly for the purpose of letting return currents
jump
between layers. I know it's a requirement for high speed signals,
especially
differrential signals. Is this also required for low speed single-ended
signals (133Mhz or slower)? If this is a requirement, what would be a
good
signal via to ground via ratio? For example, there are five signal vias
within a 1 inch area, how many ground vias do I need? Thank you Kenny
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