High-Speed Digital Signals
USB, Ethernet, HDMI, MIPI, DDR, fast SPI/QSPI. Once edges get fast, traces behave like transmission lines — impedance, return paths and length matching decide whether the link works.
Controlled impedance​
- Fast signals need a defined impedance, set by the stackup (trace width/spacing, dielectric height). Ask your fab for an impedance-controlled stackup.
- Typical targets: 50 Ω single-ended; 90 Ω (USB) / 100 Ω (Ethernet, HDMI, LVDS) differential.
- Route over a solid reference plane (usually ground) directly beneath the layer.
Return paths (the hidden half)​
- Every signal's return current flows in the plane right under it. Keep that plane continuous — do not route high-speed traces across a split/gap in the reference plane.
- Add ground stitching vias when a signal changes layers, so the return current has a path.
Differential pairs & length matching​
- Route pairs together, constant spacing; match the two halves (intra-pair skew) tightly.
- Match lengths within a bus (e.g. parallel/DDR) per the interface spec; use serpentine tuning where required.
- Minimise stubs and vias on the fast nets.
Common interfaces​
| Interface | Impedance | Notes |
|---|---|---|
| USB 2.0 | 90 Ω diff | Length-match D+/D−; keep short |
| USB 3 / PCIe | ~85–100 Ω diff | Strict; minimise vias/stubs |
| Ethernet (10/100/1000) | 100 Ω diff | Match pairs; magnetics placement |
| HDMI / LVDS / MIPI | 100 Ω diff | Tight skew; route as a group |
The #1 high-speed mistake
Crossing a plane split. A beautiful 100 Ω pair routed over a gap in the ground plane will radiate and fail EMC. Plan the stackup and plane cuts before routing.
See also​
- From Design to Delivery — request a controlled stackup
- Common Mistakes & Risks
- Reference Designs & Sources