AN 958: Board Design Guidelines

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ID 683073
Date 1/28/2022
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5.1.5.3. Skew Minimization

To minimize skew, make sure the two traces of a differential pair are equal lengths. If there is skew between pair traces and if the traces are loosely coupled, the traces can be designed as shown in Figure 26. To control the trace length, the traces separate and come back together. Because the traces are loosely coupled, the impedance is just slightly affected.

Figure 26. 45° Turns on the Serpentine Traces

When using serpentine traces, you should have 45° bends (refer to Figure 26). Figure 27 shows another example using serpentine traces. However, when using the design shown in Figure 27, make sure there is no coupling between the adjacent lines. When using serpentine traces for high-speed applications, you should avoid having the traces run parallel to each other at any point. See the example shown in Figure 26.

Figure 27. Example of Serpentine Traces

Figure 28 shows skew control for tightly coupled pairs. Because the traces are tightly coupled, the impedance changes when the traces are separated and then brought closer together. In a tightly coupled pair, skew-matching is performed at the pin level.

Figure 28. Skew Control Tightly-Coupled Differential Pairs

When designing traces on adjacent signal layers, the traces should not cross each other unless they are almost perpendicular. Parallel traces on adjacent signal layers have coupling between the traces.

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