18.104.22.168.2.7 DP/DM Trace
Place the USB PHY as close as possible to the USB 2.0 connector. The signal swing during high-speed operation on the DP/DM lines is relatively small (400 mV ± 10%), so any differential noise picked up on the twisted pair can affect the received signal. When the DP/DM traces do not have any shielding, the traces tend to behave like an antenna and picks up noise generated by the surrounding components in the environment. To minimize the effect of this behavior:
- DP/DM traces should always be matched lengths and must be no more than 4 inches in length; otherwise, the eye opening may be degraded (see Figure 7-26).
- Route DP/DM traces close together for noise rejection on differential signals, parallel to each other and within two mils in length of each other. The measurement for trace length must be started from device's balls.
- A high-speed USB connection is made through a shielded, twisted pair cable with a differential characteristic impedance of 90 Ω ±15%. In layout, the impedance of DP and DM should each be 45 Ω ± 10%.
- DP/DM traces should not have any extra components to maintain signal integrity. For example, traces cannot be routed to two USB connectors.
Figure 7-26 USB PHY Connector and Cable Connector