Differential Pair Routing for High-Speed Signals
Best practices for USB, HDMI, and LVDS layout.
Differential signaling is the backbone of modern high-speed data transfer. By sending two complementary signals (Positive and Negative) on a pair of traces, we can achieve high data rates with excellent noise immunity. However, the performance of these pairs is highly dependent on how they are routed on the PCB.
Coupling and Differential Impedance
A differential pair has two types of impedance: characteristic impedance (of each trace alone) and differential impedance (between the two traces). As you move the traces closer together, they 'couple' more strongly, which lowers the differential impedance. Most standards (USB: 90Ω, HDMI: 100Ω) assume a certain level of coupling to minimize noise.
The Rule of Equal Length (Intra-Pair Skew)
The most important rule in differential routing is length matching. If one trace is longer than the other, the two signals will arrive at different times (skew). This converts the differential signal into a common-mode signal, leading to timing errors and massive EMI emissions. Use 'serpentine' or 'trombone' routing to match lengths to within a few mils.
Avoiding Impedance Discontinuities
Every time a differential pair hits a via, a connector, or a component pad, the impedance changes. To minimize reflections, keep the traces at a constant width and spacing along the entire path. If you must use vias, use 'via stitching' to provide a return path for the ground currents nearby, maintaining signal integrity.
EMI and Common-Mode Noise
Differential pairs are inherently low-EMI because their fields cancel each other out. However, this only works if the traces are perfectly balanced. Avoid routing differential pairs near noisy components like inductors or high-speed clock lines. A continuous ground plane underneath the pair is essential for providing a stable reference and containing the electromagnetic fields.
FAQ
Should I route differential pairs as 'tightly coupled' or 'loosely coupled'?
Tightly coupled (small gap) is generally better for EMI performance and space-saving. Loosely coupled (large gap) is easier to manufacture and less sensitive to small width variations, but it requires more board area.
Can I route a differential pair over a split in the ground plane?
Absolutely not. Crossing a split in the reference plane creates a massive impedance discontinuity and causes the signal to radiate noise. Always route over a continuous, solid ground plane.
How do I calculate the gap for 100 ohm impedance?
You need to know your substrate height and dielectric constant. Our Differential Pair calculator allows you to input these variables and find the exact width and spacing required for your target impedance.