Module 07 ~45 min Interactive

PCB Layout & Cabling

Everything you learned about differential signaling only pays off if the board and cable preserve symmetry. Route the pair as a controlled-impedance, length-matched couple over a continuous ground plane, keep the TVS at the connector, then carry it out on shielded twisted pair grounded at one end. Tune a differential pair with the built-in calculator.

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Route the two lines as a coupled pair — close together, parallel, the same length — over an unbroken reference plane. The geometry (trace width W, spacing S, and height H above the plane) sets the differential impedance, which you target at ~120 Ω to match the cable and terminators.

Continuous ground plane (reference) FR-4 A B W S H
Coupled microstrip pair over a solid plane. W, S and H set the differential impedance.

An approximate edge-coupled microstrip model. Adjust the geometry to hit ~120 Ω differential. (For a real stack-up, always confirm with your fab's field solver — this is a design-intuition tool.)

Single-ended Z₀ (approx.) —
Differential Z_diff (approx.) —
Target ~120 Ω —
Tighter spacing S increases coupling and lowers Z_diff; wider traces or a thinner dielectric also lower impedance. To raise Z_diff toward 120 Ω, narrow the traces, increase spacing, or use a thicker dielectric to the plane.

✓ Length-match the pair. Keep A and B within a few mils of each other so noise couples equally and skew stays low.

✓ Keep the TVS/ESD tight to the connector. Short, wide path to the plane — before the protected boundary reaches the transceiver (Module 4).

✓ Maintain a continuous ground plane directly under the pair for the whole run.

✓ Place terminators at the true bus ends and biasing near one transceiver (Module 5).

✗ Don't cross a plane split or slot with the pair — the return current has to detour, destroying symmetry and radiating.

✗ Don't route the pair near noisy nets (switchers, clocks) or let A and B diverge around obstacles.

The single worst layout mistake for a differential interface is routing the pair across a gap in the ground plane. It converts common-mode noise rejection into common-mode noise generation.

Off the board, use shielded twisted pair (STP). The twist keeps A and B seeing identical interference (preserving common-mode rejection); the shield intercepts external fields. Match the cable's characteristic impedance (~120 Ω) to your traces and terminators.

shield GND no connect
Ground the shield at one end only to avoid a shield ground loop.
Why one end only: grounding both ends lets circulating current flow through the shield if the two grounds differ in potential — a ground loop that injects noise. Single-point grounding drains static and intercepts fields without forming that loop.

Answer to reveal instant feedback.