[SI-LIST] Re: Routing High Speed over partial GND & partial split plane references
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 12 Dec 2012 13:59:20 -0500
Not. Return currents from the high power single-ended DDR will create
common mode currents that will be injected into the differential signals
and will be a cause of increased EMI
On Wed, Dec 12, 2012 at 1:29 PM, N. Paul Taddonio <
paul.taddonio@xxxxxxxxxxxxxx> wrote:
> I think not. I would look at return currents as generating locally to the
> trace, and with equal spacing they should be equal in magnitude in the GND
> and VDD ref paths.
> They won't know that the VDD path is a bad way to go and prefer the GND
> path. The effect of the discontinuity is only reduced by half.
>
> Paul
>
>
> -----Original Message-----
> From: Rubinstein, Joseph
> Sent: Wednesday, December 12, 2012 12:58 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Routing High Speed over partial GND & partial split
> plane
> references
>
> Question:
> Everyone knows the ideal way to reference a high speed signal/diff pair is
> to have it coupled directly to a solid ground plane for the entire length
> of
> the trace. I currently have a board in CAD and this is how some of my
> routes are referenced.
>
> My stack up for example has:
>
> L5 - A solid ground plane
> L6 - Signal, with some 6G high speed differential and some DDR3.
> L7 - Power plane with splits.
>
> L6 is equal distance from L5 and L7. If L5 and L7 were both solid ground
> planes I would expect half of the return current to flow on L5 and half on
> L7.
>
> I have 6G diff pairs and DDR3 busses routed across splits on the power
> layer. My first reaction was this is a problem that needs fixing. As I
> thought about it, If the path of least resistance for the return current is
> mostly on L5, the solid ground plane, will the L7 layer not be seen as a
> discontinuity?
>
> Thoughts?
>
> Thanks
>
> Joe
>
>
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--
Scott McMorrow
Teraspeed Consulting Group LLC
16 Stormy Brook Road
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
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