[SI-LIST] Re: Common-mode return path for differential signals.
- From: steve weir <weirsi@xxxxxxxxxx>
- To: doug.hopperstad@xxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 25 Oct 2005 20:43:43 -0700
Doug,
Cross talk immunity isn't really the issue. The two nets in a pair really
can't ever get close enough that any aggressor in the same plane will
impinge anywhere close to the same amount of noise on both lines. For a
tandem aggressor, it has to run almost dead center above the pair forming
an isosceles triangle to induce balanced noise.
Manage cross talk by setting sufficient minimum distance between either
member of the pair and any aggressor.
The advantages of close coupling:
a. Better timing match. Each signal traverses a similar timing environment
until you get down into the laminate weave effects.
b. Better routing density.
c. Slightly better immunity to irregularities such as getting close to the
edge of a plane, patch or other metal.
b. can result in better cross talk for a given pitch as it increases the
distance between the two nearest lines in an agressor / victim pair.
Eric's book: "Signal Integrity Simplified" is a good reference for your
questions.
Regards,
Steve.
At 10:23 PM 10/25/2005 -0500, Doug Hopperstad wrote:
>When routing 100-ohm differential nets is there advantage to keeping the =
>two nets closely coupled other than for crosstalk issues? For current =
>designs the space between the two nets is set for 1.5 times the width of =
>the net. For a 8mil trace, the gap is 12mils (20mil pitch). If the =
>differential pair needs to become uncoupled, the trace geometry is =
>adjusted to maintain the proper impedance. Length matching maintained =
>for the route to reduce unwanted common-mode currents due to length =
>skew. All routing is done on balanced, striplines using ground reference =
>planes only.=20
>
>Some of the information I have read recently states to keep the =
>differential pair tightly coupled while others state that there is no =
>major advantage other than crosstalk. With the 12mil gap design, the =
>nets are loosely coupled (8mil dielectric thickness) and has shown good =
>results.=20
>
>Any comments or feedback would be great,
>Thanks,
>Doug.
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