[SI-LIST] Re: Current Return Vias

  • From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
  • To: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>, kenny_frohlich@xxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Mon, 24 Jul 2006 15:45:45 -0700

Ken,

Well put.  When someone makes the statement that ground vias are required
to provide a path for return currents, it needs to be accompanied with some
evidence.


> [Original Message]
> From: Ken Cantrell <Ken.Cantrell@xxxxxxxxxxxxxxxx>
> To: <kenny_frohlich@xxxxxxxxx>; <leeritchey@xxxxxxxxxxxxx>;
<si-list@xxxxxxxxxxxxx>
> Date: 7/24/2006 6:51:24 AM
> Subject: [SI-LIST] Re: Current Return Vias
>
> 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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