u-blox LARA-R203 System Integration Manual page 109

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 the 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, these being 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 59 and Figure 60 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 for a 4-layer PCB stack-up herein described, the second transmission line can be implemented
for a 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 59: Example of USB line design, with Z
L1 Copper
FR-4 dielectric
L2 Copper
Figure 60: Example of USB line design, with Z
UBX-16010573 - R12
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
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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