Published by Eric Bogatin on 09 Feb 2011
How to Eliminate Cross Talk in Differential Vias
While participating on a panel at DesignCon 2011, “How Not To Butcher Your S-Parameters”, I had the chance to chat with Brain Kirk, a Product Development Manager with Amphenol.
Of the many things I picked up, the coolest was a trick Brian mentioned that Amphenol uses in their backplane connector footprint to minimize the differential channel to channel cross talk.
Cross talk in the via field can be a dominant source of cross talk in a channel due to the proximity of long, relatively exposed vias. The way to eliminate it is to take advantage of the nature of differential signals.
Any noise that is common to both lines that make up the differential pair will exactly cancel and not contribute to differential noise. Brian pointed out that if you can arrange two pairs of vias at right angles to each other, the noise from one via in on the two vias in another pair will be equal, due to symmetry, and there will be no differential channel to channel cross talk.
His caveat was “this configuration simulates really well, but in practice there is always some asymmetry that causes more cross talk than expected.”
If you have room in your footprint to arrange the signal vias in orthogonal pairs, it might be worthwhile trying.