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How To Route USB Tracks

Geoffrey Hunter Author


The D+ and D- (data) USB traces on circuit boards require impedance controlled routing. The tolerance depends on the exact USB standard used (e.g. USB1.0, USB2.0 full-speed/high-speed, USB3.0).

The USB2.0 spec specifies a 90R differential impedance and a 45R single-ended impedance for the D+ and D- data lines.

For most normal sized PCBs, signal integrity starts becoming an issue at the 480Mb/s USB baud rate.

I recommend visiting the Transmission Lines page before continuing here...

What Is My Bandwidth?

Lets recall the well-known rule-of-thumb:

BW0.35tr\begin{align} BW \approx \frac{0.35}{t_r} \end{align}

trt_r is the maximum rise time from 10 to 90% (this is stated in the USB spec.), in seconds (s).

BWBW the resulting maximum bandwidth the track has to support, in Hertz (Hz). You can also think of this as a maximum frequency, since the bandwidth starts at 0Hz.

The USB specification states a maximum rise time of 4ns. Using the equation above, this gives us a bandwidth of approximately 87.5MHz.

From this, we can calculate the wavelength of an 87.5MHz signal travelling down a track on standard FR-4 PCB.

λ=cfϵr\begin{align} \lambda = \frac{c}{f\sqrt{\epsilon_r}} \end{align}

cc is the speed of light, in meters per second
f f is the frequency that you worked out above (which we called bandwidth)
ϵr\epsilon_r is the di-electric of the material the electro-magnetic wave is travelling through, and is unitless. In our case this will be the di-electric of the PCB. For standard FR4, this is about 4.35.

Another general rule-of-thumb is that the impedance of a PCB track is not important if it is at least 10 times smaller than the wavelength of the signal.

Thus, the minimum wavelength of the USB full-speed signal is 1.65m, and the characteristic impedance of the track is only important if the total track length is greater than 165mm.

USB StandardMinimum Rise Time, trt_rTrack Length At Which Impedance Matters
2.0 Low-speed (1.5Mb/s)75ns (USB Spec Rev 2.0, Table 7-10)3.1m
2.0 Full-speed (12Mb/s)4ns (USB Spec Rev 2.0, Table 7-9)165mm
2.0 High-speed (480Mb/s)500ps (USB Spec Rev 2.0, Table 7-8)20.5mm

As you can see from the above table, for most PCBs designs, you don't really have to worry about USB2.0 low-speed tracks, you might be concerned about really long USB2.0 full-speed tracks, and you have to control the impedance for all but the smallest USB2.0 high-speed tracks.

For more information, see the Transmission Lines page.

What Should My Impedance Be?

The impedance depends on the USB standard.

USB StandardDifferential ImpedanceSingle-ended Impedance
2.0 Full-speed (12Mb/s)90Ω±15%90 \Omega \pm 15\%
2.0 High-speed (480Mb/s)90Ω±15%90 \Omega \pm 15\% 45Ω±10%45 \Omega \pm 10\%