LISA-U2 series - System integration manual
UBX-13001118 - R27
Design-In
Page 125 of 183
C1-Public
2.2.1.3
USB signal
The LISA-U2 modules include a high-speed USB 2.0 compliant interface with a maximum throughput
of 480 Mbit/s (see Section
). Signals
USB_D+
/
USB_D-
carry the USB serial data and signaling.
The lines are used in single-ended mode for relatively low speed signaling handshake, as well as in
differential mode for fast signaling and data transfer. Characteristic impedance of
USB_D+
/
USB_D-
lines is specified by USB standard. The most important parameter is the differential characteristic
impedance applicable for 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.
•
Route
USB_D+
/
USB_D-
lines as a differential pair
•
Ensure the differential characteristic impedance (Z
0
) is as close as possible to 90
•
Ensure the common mode characteristic impedance (Z
CM
) is as close as possible to 30
•
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
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.
35
µ
m
35
µ
m
35
µ
m
35
µ
m
270
µ
m
270
µ
m
760
µ
m
L1 Copper
L3 Copper
L2 Copper
L4 Copper
FR-4 dielectric
FR-4 dielectric
FR-4 dielectric
350
µ
m 400
µ
m
400
µ
m
350
µ
m
400
µ
m
Figure 59: Example of USB line design, with Z
0
close to 90
and Z
CM
close to 30
, for the described 4-layer board layup
35
µ
m
35
µ
m
1510
µ
m
L2 Copper
L1 Copper
FR-4 dielectric
740
µ
m 410
µ
m
410
µ
m
740
µ
m
410
µ
m
Figure 60: Example of USB line design, with Z
0
close to 90
and Z
CM
close to 30
, for the described 2-layer board layup