[SI-LIST] Re: AW: Reg. GND plane requirement
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Gert Havermann" <Gert.Havermann@xxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Thu, 29 Oct 2009 15:09:26 -0700
Differential pairs work without planes all of the time. Think of the
billions of miles of UTP in the Internet! And, these paths are pretty bad.
Lee
> [Original Message]
> From: Havermann, Gert <Gert.Havermann@xxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 10/29/2009 4:09:34 AM
> Subject: [SI-LIST] AW: Reg. GND plane requirement
>
> 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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