Differential pairs are two closely spaced PCB traces that carry complementary signals with equal magnitudes and opposite polarities.
They are commonly used in high-speed interfaces such as USB, DDR, PCIe, Ethernet, HDMI, and LVDS. When designing a diff pair, you need to ensure consistent trace dimensions and adjacent continuous reference planes to reduce EMI and minimize signal skew.
Guidelines to route differential pairs
- Maintain consistent trace spacing: Keep the gap between the two traces uniform to ensure consistent coupling and differential impedance. Provide adequate spacing between differential pairs and unrelated signals to limit crosstalk.
- Match trace lengths: Keep both traces equal in length to minimize timing differences (skew). If needed, use serpentine routing to add length to the shorter trace.
- Ensure a continuous reference plane: Route differential pairs over a continuous reference plane to support stable signal transmission and minimize impedance discontinuities.
- Minimize layer transitions: Avoid unnecessary vias and maintain symmetrical transitions for both traces. If vias are required, use the same number of vias on both traces and match their structures to minimize differences in propagation delay and impedance discontinuities.
- Avoid routing across plane splits: Do not route a differential pair across gaps or splits in its reference plane, as it can increase the risk of EMI.
- Follow interface requirements: Meet the specified impedance, trace-spacing, and length-matching requirements when you’re designing connector interfaces. Consult your PCB fabricator to confirm the stack-up and routing dimensions needed to achieve the target impedance.