[SI-LIST] Re: Differential Traces and GND planes
- From: "Riley, Jonathan" <Jonathan.Riley@xxxxxxxx>
- To: "'zabinski.patrick@xxxxxxxx'" <zabinski.patrick@xxxxxxxx>, "'krishna@xxxxxxxxxxxxxxxxxxx'" <krishna@xxxxxxxxxxxxxxxxxxx>, "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 16 Mar 2012 12:41:21 +0000
In addition, even if everything is perfectly matched, cancellation only works
if the fields are identical. This is only possible when all aggressors are
infinitely far away. In reality an aggressor running parallel to the pair will
be significantly closer to one signal in the pair than to the other, so the
crosstalk induced will be worse in the nearest signal and cancellation will not
take place. Either keep a wide spacing between the pair and any possible
aggressors, or better, put a GND plane under the pair and a GND track/fill
between the aggressors and the differential pair, making sure it is well
stitched to the plane below. Do this on both sides of the pair and this then
makes it a differential coplanar waveguide. Better still, add another GND plane
above the diff pair and make sure all GNDs are stitched through.
Regards
Jonathan
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Zabinski, Patrick
Sent: 16 March 2012 12:21
To: krishna@xxxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Differential Traces and GND planes
> Differential traces perfectly cancel out the noise induced on the
trace
> provided the length of the two lines are same and are closely tied up
> all along with the controlled impedance maintained throughout the
> routing.
This is true assuming ideal conditions. However, these conditions never
exist.
For example, even if the true and complement signals are identical in
physical length, weave effect and other proximity-based features will
instill different velocities between the traces. As a result, the true
signal will arrive sooner (or later) than the complement signal, which
subsequently creates common-mode signals.
Also, it is incredibly challenging to ensure the true and complement
signals are identical in physical length. Bends/corners, vias, and
breakouts from packages are difficult to manage in board design to exact
levels. Plus, the physical length of the interconnect within the
package is often unknown to the engineer, thus it cannot be exactly
compensated for.
Beyond the interconnect, transmitters and receivers typically are
non-ideal as well. That is, they inherently inject common-mode signal
that cannot be overlooked.
In brief, differential signals and routing help along the lines you
mention, but they are not perfect. Instead, engineers must continue to
consider the common mode portion that is inherent - whether intentional
or not.
Based on this, I encourage the use of solid planes for differential
traces. Having splits in planes will create reflection, discontinuity,
and EMI/EMC issues. The magnitude of these issues might be smaller than
when using traditional single-ended signals, but the issues will exist
and need to be carefully considered.
$0.02,
Pat Zabinski
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