PCB Designer

Sunday, April 7, 2024

This App Note describes the Bandwidth Requirements of an Oscilloscope and Probes.

Source Tektronix

That's it

USB Differential Pair Length Matching

In USB design, differential pair length matching is crucial for maintaining signal integrity. Here's a breakdown of what it is and why it matters:

Differential Pairs in USB:

USB uses differential signaling where data is transmitted on two balanced lines (D+ and D-).
The difference in voltage between these lines carries the signal, making it less susceptible to noise.

Importance of Length Matching:

Ideally, both lines in a differential pair should be exactly the same length.
Any mismatch in length creates skew, which is the time difference between the arrival of signals on the two lines.
Excessive skew can degrade signal integrity, leading to errors or data corruption.

Length Matching Requirements:

The allowed skew varies depending on the USB version:
- USB 2.0 High-Speed: Maximum skew of 100 ps (picoseconds). This translates to a length difference of roughly 0.6 inches (15 mm).
- SuperSpeed USB 3.0 and above: Even tighter tolerances, with a maximum skew in the range of 30-60 ps. This translates to a much smaller length mismatch allowed, typically around 5 mils (0.13 mm).

General Guidelines:

Keep the differential pair as close together as possible throughout their routing.
Use design tools with differential pair routing features to ensure matching.
If possible, keep the total length of the pair short, especially for USB 3.0 and above.

Additional Considerations:

The length matching requirement applies to the entire signal path, including the PCB traces and any cables attached.
While length matching is important, maintaining the correct differential impedance (typically around 90 Ohms for USB) is equally crucial.

By following these guidelines for differential pair length matching, you can ensure reliable signal transmission in your USB designs.

That's it !