2 USB 2.0 Signal Integrity Challenges

USB 2.0 supports up to 480 Mbps high speed data rate along with battery charging protocols. USB 2.0 is a DC coupled system with wide signal swing. The USB 2.0 data path typically has many passive and active components such as switches, charging controllers, and protection devices. These components, though designed to be as negligible in loss to a signal’s quality as possible, can still have an effect on signals as they pass through. If these signals are already degraded in quality as is, then this small amount of loss that can be picked up from, say, a multiplexor or ESD diode, can easily push the signal to the point of being non-compliant.

Aside from these components, there are also characteristics in a board design which can attribute to the amount of loss in a signal. At the base level, the trace of a USB data path incurs some loss into the signal, with the effectiveness of the design having the ability to affect how much loss exactly. A poorly designed board may have improper routing techniques, an incorrect differential impedance due to improper width between the pair or trace width, or trace-length mismatch which will all impact the signal.

Finally, there’s also the external factors, like the cables and connectors being used. These all introduce some form of loss to the signal as they pass through them. Multiple connectors in a signal path, such as from host to device with a USB connector or board to board with a ribbon cable connector, can easily degrade the signal by a fair margin. Additionally, certain USB cables or even vendor-specific cables may have their USB 2.0 lanes routed straight rather than in a twisted pair, which directly affects the differential impedance of the USB 2.0 signal and can cause extensive signal degradation.