USB Data-Line Calculator

bit time = 1/rate. USB 2.0: one D+/D− pair, 90 Ω differential; FS/LS use ~22 Ω series, HS uses 45 Ω-to-GND termination. USB 3.x SuperSpeed: separate SSTX/SSRX pairs, 90 Ω differential, AC-coupled (~100 nF in series) and receiver-terminated — no series resistor. Gen1 = 5 Gbps, Gen2 = 10 Gbps.

When you need it: Checking bit time, differential impedance and trace length-matching when routing USB 2.0 High-Speed (480 Mbit/s) or USB 3.x SuperSpeed pairs on a PCB.

Worked example: USB 2.0 HS bit time is 1 / 480e6 = 2.08 ns; route D+/D− as a 90 Ω differential pair, length-matched within a few millimetres. USB 3.0 at 5 Gbit/s has a 200 ps bit, so low-loss laminate and tight skew matter far more.

Tips & gotchas:

• Keep D+/D− (and each SuperSpeed pair) tightly coupled, length-matched, and free of stubs or unnecessary vias.
• USB 3.x SuperSpeed pairs are AC-coupled — place the series blocking caps on the transmit side and match the pair skew.
• Put ESD protection and, if needed, a common-mode choke right at the connector, not deep in the board.
• Reference the pair over a continuous ground plane; a plane split under the pair wrecks the impedance and radiates.