USB Termination & Signal Integrity Calculator
Calculate USB bus termination resistor values, differential impedance, cable propagation delay, signal rise time, and eye opening for USB 2.0 and USB 3.0
Formula
R_term = Z₀; t_pd = L/v_prop
How It Works
USB termination is a critical aspect of signal integrity in universal serial bus (USB) communication protocols. In high-speed digital interfaces, proper termination ensures minimal signal reflection, reduces electromagnetic interference, and maintains consistent signal quality across transmission lines. The termination resistors are typically placed at the end of the communication trace to match the characteristic impedance of the transmission line, which is usually 90 ohms for USB 2.0 and USB 3.0 differential pairs.
Worked Example
Consider a USB 2.0 high-speed differential pair with a target impedance of 90 ohms. If the trace length is 10 cm and the current rise time is 500 picoseconds, the termination resistor calculation would involve determining the appropriate series and parallel termination to minimize signal reflections. By applying the transmission line impedance formula and considering the signal characteristics, an engineer would select precision 45-ohm resistors to create a parallel termination network that matches the line impedance and reduces signal discontinuities.
Practical Tips
- ✓Always use precision resistors with tight tolerance (1% or better) for USB termination
- ✓Place termination resistors as close to the connector or transceiver as possible
- ✓Consider differential and common-mode termination strategies
- ✓Use proper PCB layout techniques to minimize signal reflections
Common Mistakes
- ✗Using incorrect resistance values for the specific USB standard
- ✗Placing termination resistors too far from the signal source
- ✗Neglecting the impact of trace geometry on signal integrity
Frequently Asked Questions
Why is USB termination important?
USB termination prevents signal reflections, reduces electromagnetic interference, and ensures reliable high-speed data transmission by matching transmission line impedance.
What is the typical termination impedance for USB?
For USB 2.0 and USB 3.0 high-speed differential pairs, the typical termination impedance is 90 ohms.
How do I select the right termination resistors?
Choose precision resistors with tight tolerance, consider the specific USB standard, and match the characteristic impedance of the transmission line.
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