[SI-LIST] Re: Reference plane for differential pairs
- From: "Gourari, Alexandre" <Alexandre.Gourari@xxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 17 Sep 2004 19:31:12 +0100
IMHO unless your major concern is crosstalk between whatever your
differential pairs carry and whatever your 12V rail powers you be better off
using 12V plane to close your diff pair common mode noise return path. It
will always be better for EMI and pair-pair xtalk.
The trick is you don't have much control over that for high frequency due to
inherent interplane capacitance, but if you want to improve on that toward
lower frequency you need to throw in some decoupling caps like Steve
suggested. I assume your backplane stack-up is defined and you can't play
with it.
Best regards,
Alex Gourari,
-----Original Message-----
From: steve weir [mailto:weirsp@xxxxxxxxxx]
Sent: Friday, September 17, 2004 1:12 PM
To: David.sanderson@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Reference plane for differential pairs
David, since the 12V is unlikely to be tightly coupled to the logic ground
near either the source or the load, you should avoid using it as a return
reference. If you are really hard-up, then you might get away with adding
LOTS of decoupling capacitors or very low inductance capacitors such as X2Y
or IDC especially near the connectors, and switching the return reference
from logic ground on the connector side to 12V plane on the
backplane. But, I would still discourage this.
Steve
At 10:49 AM 9/17/2004 +0100, David Sanderson wrote:
>I am a little confused as regards reference planes and differential pairs
>(SATA in this case).
>I am routing out differential signals from a controller board to a
>backplane. The backplane has a couple of 12V planes. Can I use these as a
>reference plane for my differential pairs? There are no (significant) high
>frequency decoupling capacitors between 12V and 0v in the system. My
>feeling is that, yes the 12V plane will give me the 100 ohm differential
>impedance (assuming 50 ohm single ended to each trace), but what about the
>return path for any current imbalance? Can it be fixed by putting down
>some high frequency decoupling caps somewhere on the backplane, probably
>close to the connectors at each end of the link?
>
>Or would it be best to avoid using the 12V plane as a reference?
>
>It is somewhat related to a question a day or two ago asking whether a
>copper plane beneath differential pairs actually needs to be connected to
>anything. I think the consensus was, theoretically no, because net current
>should be zero, however, imbalances will always occur, so there will
>probably be a small current flowing from/to the drivers and hence the
>plane should be connected to ground to complete the path.
>
>Thanks in advance for any words of wisdom.
>
>David Sanderson
>
>
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