PCB design glossary
Differential Pair
AKA: diff pair, differential pairs
Two traces routed together that carry equal-and-opposite signals — used for high-speed differential signaling.
Differential pairs (USB, HDMI, LVDS, Ethernet) carry a signal as two equal-and-opposite traces. The receiver looks at the difference between them, which makes the signal highly immune to common-mode noise.
Routing differential pairs requires matched lengths, controlled impedance (usually 90 Ω or 100 Ω), and careful trace spacing. Most EDA tools, including PCBEditor, have a dedicated differential-pair routing mode.
Why signals are sent as pairs
The receiver reads the difference between the two lines, not their voltage against ground. Noise that couples onto both lines equally cancels out, and the pair's fields largely cancel too, so it radiates less. That makes differential signalling the choice for fast, reliable links.
Common interfaces and their impedance
Hitting the impedance depends on trace width, spacing between the two traces and the stack-up. Use your fab's stack-up data and an impedance calculator.
- USB 2.0: 90 Ω differential.
- Ethernet: 100 Ω differential.
- HDMI, LVDS and many other high-speed links: 100 Ω differential.
Routing rules
- Keep the two traces together with constant spacing along their whole length.
- Match their lengths, adding small serpentines to the shorter one where needed.
- Route over a solid ground plane and never across a split.
- Keep vias few, and use them in pairs when the traces change layer.
- Keep other signals away, typically several trace widths.
Work it out with a free tool
What spacing gives 90 ohms?
Enter trace width, gap and dielectric and the differential pair calculator gives the impedance for USB, Ethernet, PCIe and LVDS targets.
Calculate pair impedance