[SI-LIST] Re: Current Return Vias

  • From: "Giovanni Guasti" <giovanni.guasti@xxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Sat, 22 Jul 2006 20:38:30 +0100

Kenny,
GND vias near the signal are not dedicated to return currents, but they
are often used to optimize the impedance of the via.
As the via is a short transmission line, only the higher speed signals
can benefit of the difference between an optimized via and a "usual"
via.

You have to compare the higher frequency component of your signal, its
wavelength and the via length. This will give you an idea of the
effective needing to optimize this short transmission line.

Of course you could have a 133MHz signal with very sharp edges and high
frequency components, even if it seems very unusual... In this case it
would be wise to choose a slower transmitter!

The rule is to understand if the via behaves like a transmission line
for your signal or not, and in the first case to do the best to reduce
impedance discontinuities.
Best regards,
        Giovanni


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Lee Ritchey
Sent: 22 July 2006 18:36
To: Kenny Frohlich; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Current Return Vias

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 <kenny_frohlich@xxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> 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)? =20
>   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?
>   =20
>   Thank you
>   Kenny
>   =20
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