Anti Pads in Differential Pairs

Hello Experts,
The benefits of antipads in high speed differential pairs signals are known.

But I see in the industry some layouts that put the antipads in all
differential pairs, including low frequency clocks and fast ethernet
signals.

In this cases, the "ground starvation" will result more issues in the
signal than the benefits of antipads ? What do you all think about it?

Sérgio



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SERGIO Eduardo Spader
DATACOM
System Architect - Ethernet Switches
Av. França, 735 - Porto Alegre, RS - 90230-220
DDR: 51 3933 3234
Skype: datacom_sergio
e-mail: sergio@xxxxxxxxxxxxxx
site: www.datacom.ind.br

sergio 5 years 3 months 26 days

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Answered byChris.Cotton 5 years 3 months 26 days
Hi Sergio,

I think you may be confusing the intended purpose of antipads. Antipads have
nothing inherently to do with signal integrity or high speed signaling. Antipds
are merely the spacing from the drill/pad to power/ground plane metal to avoid
short circuiting once the via hole is drilled and plated. An antipad is
generally needed for all drilled and plated holes. Classically the trade-off
was between manufacturability of the board by the fabricator to achieve high
yields and not creating too many void areas in the ground/power plane

As signal frequencies increase, the *shape* and size of the antipad does affect
electrical performance. But in these cases there is no option to remove the
antipads. The options are just to change the size and/or shape of the antipad
to maximize electrical performance and in some cases minimize "ground
starvation" as you called it.

Chris

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