12.1 Layout Guidelines

TI recommends using at a minimum a four-layer stack-up to accomplish a low-EMI PCB design.

• It is important to match the electrical length of these high-speed traces to minimize both inter-pair and intrapair skew.
• Placing a solid ground plane next to the high-speed signal layer establishes controlled impedance for transmission line interconnects and provides an excellent low-inductance path for the return current flow.
• Placing the power plane next to the ground plane creates additional high-frequency bypass capacitance.
• Routing the slower speed control signals on the bottom layer allows for greater flexibility as these signal links usually have margin to tolerate discontinuities such as vias.
• If an additional supply voltage plane or signal layer is needed, add a second power / ground plane system to the stack to keep it symmetrical. This makes the stack mechanically stable and prevents it from warping. Also the power and ground plane of each power system can be placed closer together, thus increasing the high frequency bypass capacitance significantly

Figure 40. PCB Stack

12.2 Layout Example

Figure 41. Layout Example