Ever wonder the effects of shared anti-pads on differential signals?

Studies show that the benefit using shared anti-pads in this way is reduced capacitance load to planes. What has not been fully discussed is whether the benefit holds when differential traces are connected to the vias that use shared anti-pads; in other words, what would the signal quality be when differential signals go through a pair of differential vias with shared anti-pads? Figure 2 shows the configuration of such cases.

One thing to pay attention to is the area under the feeding traces that connect to the differential vias. We can compare this area in the shared anti-pad configuration with the one using separate anti-pads shown in Figure 3. The shared anti-pad takes away part of via coupling to planes, which reduces the capacitance between via barrow and planes; however, it also takes away the reference plane, or the return path, of the feeding traces when they are approaching the connection points to vias. Of course, the effect can be less significant in the case of tightly coupled differential, but it will always be there.

Next, let’s  take a look at the S-parameter models extracted from the two structures in Figure 2 and 3. Both of them have the same geometry parameters, except one uses separate anti-pads, and the other shares anti-pads in a shape close to “Oblong”. The 12 layer stackup is shown in Figure 4. All the inner layers are planes for the purpose of study.

Figure 4 Stackup of a 12-layer design

Figure 5 provides comparison of insertion loss from the two structures. We can see that both S-parameter models behave very similarly before the first resonance point; but the shared anti-pads increase the loss a little in the frequency range of 2GHz to 8GHz, because of the reference plane reduction, while they help shift the resonant peak further to higher frequencies (from 18GHz to 18.5GHz) due to the reduced capacitance load.

Figure 5 Comparing insertion loss (12 layer design) — Purple: shared anti-pads; Blue: separated round anti-pads

Another test case with fewer layers reveals similar results. Figure 6 shows comparisons of S-parameters of a 6 layer design. Shared anti-pads do not show their benefits until via-to-plane capacitance starts being effective; before that happens, using separate anti-pads actually provides better signal quality.

Figure 6 Comparing insertion loss (6 layer design), Purple and Green: shared anti-pads, Blue and Yellow-: separated round anti-pads