[SI-LIST] Re: AW: Re: Routing differential lines as single ended traces ?
- From: John Ellis <John.Ellis@xxxxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 26 Sep 2011 05:43:51 -0700
Hi Jerry,
Routing the differential pair as two single ended lines has advantages in DDR
memory interfaces that are often routed in microstrip. In microstrip coupled
signals will travel at different velocities depending on activity of the
neighbors. If the neighbors are quiet, a signal will travel with a velocity, V.
If the neighbors switch in the opposite direction, the velocity will be faster
since odd mode coupling will be dominant and air will play a large part in
determining the effective dielectric constant. If the neighbors switch in the
same direction, the dominant mode will be even coupling which largely confines
the fields to the dielectric beneath the traces. This results in a slower
signal velocity.
A DDR memory interface is source synchronous. The differential strobe must be
placed in the center of the eye formed by the data signals under even, odd and
uncoupled switching conditions. When a tightly coupled differential pair is
used for the strobe and routed in microstrip, this will always travel with the
faster odd mode velocity. Consequently, this will cause the strobe to arrive
earlier, reducing set up margins. Depending on the length of the interface,
this can result in 10's of ps of lost margin if not compensated for. Routing
the differential strobe as two uncoupled single-ended lines should eliminate
this effect. The two legs must, of course, be tightly skew matched to yield a
monotonic differential strobe.
John Ellis
Sr. Staff R&D Engineer
Mixed Signals &
I/O Libraries Group
Synopsys, Inc.
email: jellis@xxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Havermann, Gert
Sent: Monday, September 26, 2011 3:51 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] AW: Re: Routing differential lines as single ended traces ?
Hi Jerry,
on validation platforms or test fixtures it is beneficial to route single ended
instead of coupled. The reasons are, that the test equipment is single ended,
and that you don't want any PCB effects to mask the DUTs performance.
In Lab environments, you can live with the slightly worse noise performance (in
the lab you will hardly find any performance degradation unless you inject
noise).
I wouldn't use this scheme for anything else but testing as it needs too much
space on the PCB.
BR
Gert
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-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von steve weir
Gesendet: Montag, 26. September 2011 04:23
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Re: Routing differential lines as single ended traces ?
On 9/25/2011 6:57 PM, Low Jerry wrote:
> Hi,
> I recently came across a validation platform where the LVDS
> differential lines are routed as single ended traces traces instead of
> tightly coupled differential pairs. The motivation I found was so that
> they could use each of the pairs as single ended traces when needed as
> well. When I probed deeper it seems like the person who proposed this
> scheme has left. So I would like to seek help here on some
> clarification
>
> - Will a scheme like this impact the performance of the
> differential pair
The scheme itself will not.
> ? Since I understand that differential routing is more immune to noise.
That is largely a myth. There are only a few very special circumstances where
tightly coupled pairs exhibit better noise rejection to PCB trace noise
aggressors than reasonably routed, loosely coupled pairs.
> - What are the considerations/feasiblity studies that need to be done
> before implementing a scheme like this ?
They are the same as with any signal integrity requirements. The scheme
doesn't impose anything extra. It does remove some headaches.
> - Since this is a validation platform what measures can be taken ensure
> that the performance seen are similar to a production platform if the
> production platform is routed in differential.
Homework gets done or it doesn't.
Steve
>
> Thanks in advance for the feedbacks. Have a great day.
>
>
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