250 www.xilinx.com Virtex-4 RocketIO MGT User GuideUG076 (v4.1) November 2, 2008Chapter 11: Design of Transitions RDifferential ViasThe most common transition is the differential via where the signal pair must transitionfrom an upper stripline layer or top microstrip to a lower stripline layer or bottommicrostrip.Figure 11-10 shows a Ground-Signal-Signal-Ground (GSSG) type differential via. Groundvias are connected to each ground plane in the stack-up, while signal layers only containpads for the entry and exit layers and top and bottom pads needed for manufacturability.A key advantage of a GSSG via is that it allows for the differential signal’s common-modecurrent to flow in the ground via near the corresponding signal via, reducing excessinductance. The signal path is also symmetrical between the P and N halves of thedifferential signal, which minimizes differential-to-common-mode conversion due to P/Nimbalance.A good starting point is to use the dimensions shown in Figure 11-10 as an exampledifferential via design for an 80 mil thick board. To accommodate density constraints, thedimensions can be scaled accordingly to preserve the ratios of each dimension relative tothe others. Such scaling preserves much of the impedance performance of the differentialvia while allowing variation in overall size to better suit specific applications. These finaldimensions are limited by manufacturability and density constraints.Where possible, the via stub lengths should be kept to a minimum so that the signal cantraverse through the entire length of the via. Figure 11-11 shows models of differential viaswith moderate stubs below layer 11 (left side) and long stubs below layer 6 (right side). Theanalysis results of these models is shown in Figure 11-12, which compares the S-parameterreturn loss for common-mode (SCC11) and differential (SDD11) responses.Figure 11-10: Differential Via Design ExampleUG072_c3_32_102805Via Diameter = 12 mils (0.012 inches)Pad Diameter = 22 milsAnnular Ring = 5 milsGSSG Via Pitch = 40 milsOblong Antipads = ~55 mils x 95 mils,aligned with ground pads
