Carson
With all things SI, it depends. It depends on the number of differential
pairs, the density of differential pair vias, and the data rate.
Since vias are generally not long enough for crosstalk saturation to occur,
increasing data rate from 10 Gbps to 25 Gbps increases crosstalk by 2.5x.
There is no rule, but I generally recommend at least a ground via per signal
via within a 2 mm distance or less (1 mm to be preferred) without any EM
analysis. What is extremely important is to not cause sharing of a ground via
between two differential pairs, when that is the only via around. It is a
recipe for high crosstalk.
Scott McMorrow, CTO Signal Integrity Group
Samtec
Office 401-284-1827 | +1-800-726-8329
www.samtec.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Carson Au
Sent: Wednesday, May 31, 2017 3:52 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Return Path Vias
The perennial debate... on SI-List.
SERDES differential channels 10Gbps+. For layer transitions, is it OK to use
just one return path via (both sides of the differential via sharing one GND
return path via, symmetrical/balanced)? Is it OK to use no return path vias at
all (Lee Ritchey advocates for this)?
Is there literature comparing 2x, 1x and NO return path vias?
I can imagine that the common mode conversion would be an issue. But how much
of an issue?
Anecdotal stories that one method "just works" is not sufficient proof (i.e.
for the no return vias case) I have been able to get a 10Gbps+ channel working
with only one side of the differential connected - with a BER better than
10e-15 (ran overnight with no bit-errors)!! Yet none would say this channel is
well designed.
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