[SI-LIST] Reference plane for differential pairs
- From: "David Sanderson" <David.sanderson@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 17 Sep 2004 10:49:38 +0100
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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