As with any high-speed design, there are many factors influencing the overall performance. The following list indicates the critical areas to consider during the design process:
- Use 85Ω impedance traces. Perform length matching on the P and N traces on the single-ended segments of the differential pair.
- Use a uniform trace width and spacing for differential pairs.
- Place AC-coupling capacitors near to the receiver end of each channel segment to minimize reflections.
- For Gen 3.0, 4.0, and 5.0, AC-coupling capacitors of 220nF are recommended with a maximum body size of 0402 and a cutout void on the GND plane below the landing pad of the capacitor to reduce parasitic capacitance to GND.
- Back-drill connector vias and signal vias to minimize stub length.
- Use reference plane vias to ensure a low inductance path for the return current.