Issue link: https://iconnect007.uberflip.com/i/952929
44 DESIGN007 MAGAZINE I MARCH 2018 to estimate the impedance, to the lower plane on layer 3, by swapping the layer 2 plane for a signal layer as depicted. This increases the impedance from 72 to 100Ω. The four ground vias near the AC coupling capacitors and the two on the connector lands, in Figure 3, are essential to provide a direct current return path to the lower GND plane and back. Vias correctly placed will serve to minimize crosstalk and contain the common modes that propagate due to signal skew. Com- mon mode conversion near the receiver can have some disastrous multi-aggressor crosstalk peaking implications. Figure 5 shows a TDR impedance plot from an Ansoft HFSS simulation. The red line is the Figure 4: 100Ω impedance when the plane beneath the capacitor is removed (source: iCD Stackup Planner). Figure 5: TDR simulation of impedance profile vs plane cut out width (source: Altera).