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Differential pairs are an important technique in PCB design for transmitting high-speed digital and analog signals with a high degree of immunity to noise and interference. They consist of two conductive traces (signal and its inverse), designed to carry equal and opposite signals. The receiving circuit detects the difference between the two signals, so common noise induced on both lines can be rejected.

When routing differential pairs on a PCB, it's crucial to adhere to certain guidelines to ensure signal integrity. Here are some key points to consider:

Trace Length Matching

The two traces in a differential pair should be the same length to prevent timing skew between the signals. Timing skew can degrade the performance of the differential pair by causing the receiver to incorrectly interpret the difference between the signals.

Spacing Between Traces (Coupling)

The two traces should maintain a constant separation along their length to maintain consistent impedance. This is often called "edge-coupled" or "over/under-coupled" in a multilayer board. The spacing depends on the differential impedance required by the design, typically between 80 and 120 ohms for most differential pair applications.

Routing on the Same Layer

The two traces of a differential pair should be routed on the same layer of the PCB. This is to ensure that they are exposed to the same electrical environment, thereby ensuring that any noise is induced equally in both traces.

Avoiding Right Angles

As with any high-speed signal, it's best to avoid routing differential pairs with 90-degree angles, as they can cause impedance changes and signal reflections. Use 45-degree or curved bends instead.

Minimize Vias and Other Discontinuities

Each via or other discontinuity (like a change in trace width) can cause a change in impedance, potentially leading to signal reflection. It's best to minimize these where possible.

Guard Traces

To protect differential pairs from crosstalk, it's often a good idea to route them with "guard traces" - ground traces on either side of the pair. The ground traces act as a barrier to prevent signals in nearby traces from interfering with the differential pair.

Differential Termination

It’s important to correctly terminate the differential pair to prevent signal reflections. The termination resistance should match the differential impedance of the pair.

Remember, these are guidelines and not strict rules. Each design might need specific tweaks depending on various factors. Also, make use of the Differential Pair Routing feature available in most advanced PCB design software, which helps to maintain the required spacing and length equality automatically.