What are the best practices for routing differential pairs on a high-speed PCB?

High-speed PCB design is a challenging and rewarding skill for hardware developers. It requires careful planning, testing, and optimization of the circuit layout and components. One of the most critical aspects of high-speed PCB design is routing differential pairs, which are two traces that carry complementary signals with opposite polarity. Differential pairs can reduce noise, crosstalk, and electromagnetic interference, but they also have some special requirements and best practices. In this article, we will explore some of the key tips and tricks for routing differential pairs on a high-speed PCB.

1 Choose the right impedance

The impedance of a differential pair is the combined resistance, capacitance, and inductance of the two traces. It affects the signal integrity, power consumption, and performance of the circuit. The impedance of a differential pair should match the impedance of the source and load devices, as well as the transmission line. This will minimize reflections, distortions, and losses. To achieve a consistent impedance, you need to consider the trace width, spacing, thickness, dielectric material, and layer stackup of your PCB. You can use online calculators, simulation tools, or design rules to help you determine the optimal impedance for your differential pair.

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2 Maintain the same length

The length of a differential pair is another important factor for signal quality. The length of the two traces should be as close as possible, ideally within 5 mils or less. This will ensure that the signals arrive at the same time at the destination, avoiding phase shifts, skew, and timing errors. To maintain the same length, you need to avoid unnecessary bends, corners, or vias in your differential pair. If you need to adjust the length, you can use meanders, serpentine patterns, or trombone shapes to add or remove some distance. However, you should avoid creating sharp angles or abrupt changes in direction, as they can cause impedance discontinuities and signal reflections.

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Tyler Reid

Senior RF Electrical Engineer at Kyocera International, Inc. - Semiconductor Components Group
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The accuracy of the length match required between each line in the pair will depend on the maximum frequency of operation. Higher frequencies or data rates will require tighter matching.

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3 Avoid parallel coupling

Parallel coupling is the phenomenon of unwanted coupling between two adjacent differential pairs. It can cause crosstalk, interference, and signal degradation. To avoid parallel coupling, you need to increase the spacing between different differential pairs, especially if they operate at similar frequencies or have different impedances. You can also use ground planes, shielding traces, or guard traces to isolate and protect your differential pairs from each other. Alternatively, you can use orthogonal routing, which means routing your differential pairs at 90 degrees to each other on different layers. This will reduce the overlap and coupling area between them.

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4 Follow the return path

The return path is the path that the current takes to return to the source after passing through the load. It is essential to maintain a low and continuous return path for your differential pair, as it affects the signal integrity, noise immunity, and electromagnetic compatibility of your circuit. The return path should follow the same route as the differential pair, as close as possible, and on the same layer if possible. You can use ground planes, power planes, or ground vias to provide a solid and stable return path for your differential pair. You should avoid crossing gaps, splits, or discontinuities in your planes, as they can create loops, parasitics, or reflections in your return path.

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Tyler Reid

Senior RF Electrical Engineer at Kyocera International, Inc. - Semiconductor Components Group
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The ideal return path would be ground traces on both sides of the pair with solid ground planes above and below the differential pair connected with vias along the entire path.

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5 Test and verify

The final and most important tip for routing differential pairs on a high-speed PCB is to test and verify your design. You can use simulation tools, such as SPICE or SI, to analyze and optimize your differential pair performance, impedance, crosstalk, and signal integrity. You can also use measurement tools, such as oscilloscopes or network analyzers, to validate and debug your differential pair functionality, timing, and quality. You should test and verify your design at different stages of the PCB development process, such as schematic capture, layout design, prototype fabrication, and final production.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the best practices for routing differential pairs on a high-speed PCB?

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1 Choose the right impedance

2 Maintain the same length

3 Avoid parallel coupling

4 Follow the return path

5 Test and verify

6 Here’s what else to consider

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