[SI-LIST] AW: Reg. GND plane requirement

  • From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 29 Oct 2009 10:09:24 +0100

in theory, for ideal differential transmissions, a differential pair will work 
without GND-planes. Reality is different though.
Think of it this way: For a good signal transmission you need impedance 
controlled traces. You don't want discontinuities on your line (especially at 
3GHz speeds). Looking at one single stand alone differential pair on a PCB  
(there is nothing else on and around the PCB. Thats the theory I mentioned. in 
reality you have other traces, components copper pads heat sinks.... Every 
peace of conductive material in close proximity to your diff. Pair will affect 
its impedance. You will have dozens of discontinuities, doesn't sound like a 
transmission line, does it? Metal planes above and underneath the pair helps to 
control the impedance. You May think using coplanar coupled waveguide routing 
can do the job too, but thats not true. You will still have the impact of all 
conductive materials of the other layer, the impact is just a little less than 
without any GND-plane.

Another Reality thing is, that there is no 100% symmetrie of a differential 
pait, thus there will be common mode currents generating common mode return 
currents. If you don't provide a GND-return path for these signals, you easily 
run into SI-trouble.

My short answer is: Yes, GND/VCC planes are required above and below the 
routing layer.

PS: si-list doesn't support attachments. You can draw things in ASCII, or 
upload it to some sharing place.

BR
Gert


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-----Ursprüngliche Nachricht-----

Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im 
Auftrag von chundi srikanth
Gesendet: Donnerstag, 29. Oktober 2009 09:06
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Reg. GND plane requirement

Hi Techies,
I have a query for you. We are designing a 12-layer mixed signal board. In 
which High-speed, ADC's are connected to FPGA through LVDS differential pairs. 
In PCB layout is it really required to have a GND/VCCV plane above and below 
the plane in which the differential are been routed? If it is not required then 
What about the cross talk effect? How can we over come that?

The LVDS signals are running at 122.88MHz, 245.76MHz and 983.04Mhz and 3GHz 
speed. And there are few main clocks available i these signals too.

Please let me know your views on this. Please see the attachment for my 
proposed stack up details.


Thanks & Regards
Srikanth Chundi



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