[SI-LIST] Re: Current Return Vias
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Joel Brown" <joel@xxxxxxxxxx>
- Date: Mon, 24 Jul 2006 15:44:09 -0700
Joel,
First, you should have stitched your ground planes to each other every
where there is a ground pin on a device, so these ground planes should be
well tied to each other every place you have a component whether it be a
bypass capacitor or an IC. (This argues for spreading your bypass
capacitors evenly around the planes as has been advocated by a number of
members of this forum in the past.) Only where you might change layers in
an empty part of the PCB is this not so. Second, when you engineer your PDS
you need to insure that the impedance is a few milliohms across all of the
frequencies in your signals, so the Vdd planes are "shorted" to these
ground planes by this method. ( To achieve this low impedance across the
broad range of frequencies in your signals, you will be using plane
capacitance as well as a variety of different values of capacitors. When
done right, all of the planes are "shorted" to each other and return
currents have all the paths they need. If the PDS isn't properly designed
you can wind up with EMI problems and unstable logic operation.
> [Original Message]
> From: Joel Brown <joel@xxxxxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>
> Cc: Kenny Frohlich <kenny_frohlich@xxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 7/23/2006 12:25:04 PM
> 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.
> 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
method 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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