[SI-LIST] Diff stripline, split in one of the gnd planes acceptable?
- From: "Morten Leikvoll" <nerd@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 8 Oct 2009 13:18:09 +0200
I got a sandwich config of diff stripline (lvds):
1:lvds (microstrip)
2:GND
3:lvds (diff stripline)
4:GND
5:lvds (diff stripline)
6:GND
...
For microstrip I see the importance of keeping the 2:GND plane unsplit for
common mode return currents, but for normal stripline there are two
(2:GND,4:GND) return paths for the 3:lvds pair.
I wonder if its ok to make split in 4:GND without disturbing the impedance
noticeably. I basically want to share the 4:GND plane with areas of power,
and it will cause gnd/power splits. There will be no signal nets there, just
low impedance like mixed power and GND.
Should I worried about the return paths if I do add splits?
Should I worry abut reflections if the gap caused by impedance disturbance?
Maybe gaps could be "mended" with a MLCC just over/under each pair?
I got rather long distances, maybe up to 50cm but breakouts/vias only close
to both ends, and the bitrate is 1GHz.. (Altera StratixIII in both ends)
If any doubts, the layout geometry terms are taken from here:
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=util_pcb_imp_calculator
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