[SI-LIST] Re: Current Return Vias

  • From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
  • To: <kenny_frohlich@xxxxxxxxx>, <leeritchey@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 24 Jul 2006 07:48:39 -0600

Kenny,
At 133MHz, and assuming your edge rates are > 500ps, you aren't going to
need them, even if you have twice as many layers.  It's an important effect,
but at 133MHz it's not relevant.  What is important, as Lee indicated, is
your PDS design.

Ken

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Kenny Frohlich
Sent: Saturday, July 22, 2006 1:56 PM
To: leeritchey@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Current Return Vias


Lee,
  I'm not talking about jumping from one layer to the next adjacent layer
which shares the same referrence plane.  In this case, I do not need a
ground via.  But I'm asking about jumping from one layer to another layer
that has a differrent reference ground plane.  For example, on an 8 layer
PCB stackup where the two ground planes are layers 2 and 7,  the signal
jumps from the top layer (layer 1) to the bottom layer (layer 8).

  Thank you
  Kenny
Lee Ritchey <leeritchey@xxxxxxxxxxxxx> wrote:
  Kenny,

It is not true that you need a "return current" via next 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 have a 4 layer PCB, such as the mother board in a PC, where
there are only two planes, 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 take care of themselves. EMI is minimized
he same way..




> [Original Message]
> From: Kenny Frohlich
> To:
> 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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