Xilinx Virtex-5 RocketIO GTP User Manual page 244

Chapter 13: Design of Transitions
A key advantage of a GSSG via is that it allows for the signal's return current to flow in the
ground via near the corresponding signal via, reducing excess inductance. The signal path
is also symmetrical between the P and N halves of the differential signal, which is critical
in controlling common-mode artifacts due to P/N imbalance.
The larger oblong antipads reduce excess fringing capacitance between the via body and
the surrounding planes edges. Unused pads are also removed.
A good starting point is to use the dimensions shown in
differential via design for an 80 mil board. To accommodate density constraints or the lack
thereof, the dimensions can be scaled accordingly to preserve the ratios of each dimension
relative to the others. Such scaling preserves the impedance performance of the differential
via while allowing variation in overall size to better suit specific applications. These final
dimensions are limited by manufacturability and density constraints.
While the via length can be varied by a small amount to suit boards that are thicker or
thinner than the 80 mil example, changing the ratio of the via length relative to other
dimensions affects the via's impedance. For this and other configurations of differential
vias, it is best to simulate a model using 3D field solver tools to ensure that performance
targets are met.
244
Via Diameter = 12 mils (0.012 inches)
Pad Diameter = 22 mils
Annular Ring = 5 mils
GSSG Via Pitch = 40 mils
Oblong Antipads = ~55 mils x 95 mils,
aligned with ground pads
Figure 13-13: Differential Via Design Example
www.xilinx.com
Figure 13-13 as an example
Virtex-5 RocketIO GTP Transceiver User Guide
UG196 (v1.3) May 25, 2007
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UG196_c13_13_051406