u-blox LARA-R2 series System Integration Manual page 106

2.6.2.2 Guidelines for USB layout design
The USB_D+ / USB_D- lines require accurate layout design to achieve reliable signaling at the high speed data
rate (up to 480 Mb/s) supported by the USB serial interface.
The characteristic impedance of the USB_D+ / USB_D- lines is specified by the Universal Serial Bus Revision 2.0
specification [9]. The most important parameter is the differential characteristic impedance applicable for the
odd-mode electromagnetic field, which should be as close as possible to 90  differential. Signal integrity may
be degraded if PCB layout is not optimal, especially when the USB signaling lines are very long.
Use the following general routing guidelines to minimize signal quality problems:

Route USB_D+ / USB_D- lines as a differential pair.

Route USB_D+ / USB_D- lines as short as possible.

Ensure the differential characteristic impedance (Z

Ensure the common mode characteristic impedance (Z

Consider design rules for USB_D+ / USB_D- similar to RF transmission lines, being them coupled differential
micro-strip or buried stripline: avoid any stubs, abrupt change of layout, and route on clear PCB area.

Avoid coupling with any RF line or sensitive analog inputs, since the signals can cause the radiation of some
harmonics of the digital data frequency.
Figure 57 and Figure 58 provide two examples of coplanar waveguide designs with differential characteristic
impedance close to 90  and common mode characteristic impedance close to 30 . The first transmission line
can be implemented in case of 4-layer PCB stack-up herein described, the second transmission line can be
implemented in case of 2-layer PCB stack-up herein described.
L1 Copper
FR-4 dielectric
L2 Copper
FR-4 dielectric
L3 Copper
FR-4 dielectric
L4 Copper
Figure 57: Example of USB line design, with Z
L1 Copper
FR-4 dielectric
L2 Copper
Figure 58: Example of USB line design, with Z
UBX-16010573 - R02
400 µm 350 µm 400 µm 350 µm 400 µm
close to 90  and Z
0
410 µm 740 µm 410 µm 740 µm 410 µm
close to 90  and Z
0
Objective Specification
LARA-R2 series - System Integration Manual
) is as close as possible to 90 .
0
) is as close as possible to 30 .
CM
close to 30 , for the described 4-layer board layup
CM
close to 30 , for the described 2-layer board layup
CM
35 µm
270 µm
35 µm
760 µm
35 µm
270 µm
35 µm
35 µm
1510 µm
35 µm
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