[SI-LIST] Re: AW: Re: Reference layers for high speed diff pairs
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: jeff.loyer@xxxxxxxxx
- Date: Wed, 16 Jan 2013 11:52:22 -0500
On Wed, Jan 16, 2013 at 11:28 AM, Loyer, Jeff <jeff.loyer@xxxxxxxxx> wrote:
> In my experience in server designs, there is virtually zero impedance
> between VDDQ and GND planes, even on a bare board. Shooting a TDR between
> them, you see a dead short (unless you wait a very long time in which case
> you'll see the classic charging of a capacitor). Thus, it doesn't matter
> which plane you choose to reference your signal to -
>
>
Jeff, this only holds true below the power system parallel resonant
frequency (PRF). For single ended low speed signals, such as DDR, you may
be correct (as long as the signal bandwidth is below PRF). However, it is
pretty much impossible to design a power delivery system with a PRF high
enough to support common SERDES switching rates. It is in these cases with
a high Nyquist frequency that dual-referenced or power plane only
referenced signals should be avoided.
As a pretty decent rule of thumb, I would say that dual-referenced signals
might be justified to a bandwidth of approximately 1 octave below power
system PRF (PRF/2), but not without additional noise, since the power
impedance is on a rising inductive slope at this point. Above that point, I
would not dare run diff pairs with anything but ground.
regards,
Scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
16 Stormy Brook Road
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
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