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Table 42: Routing Constraints when Configured as a Device (Continued)
Par a meter
CM1 Common mode impedance @ 500 MHz
CM1 DC resistance
NOTES
1. The target impedance should be 90 ohms differential; any mismatch should be taken as a part of the
loss.
2. In the USB routing, it is essential to avoid capacitive discontinuities and via through-hole long stubs.
Via stubs can occur on vias or on the USB connector pins. For further information refer to
Generic Guidelines for SERDES Interfaces, on page
3. Discontinuities, such as capacitive type, can cause significant insertion loss degradation, causing the
assumption, taken as loss per inch, to be smaller than actual.
4. In BGA packages, some package length routing skew may exist and help compensate for the ball
position within the package.
5. The target impedance priority is to match the differential impedance of these signal traces and adjust
the single-ended impedance and the common mode impedance, accordingly.
6. The tolerance is manufacturing tolerance.
7. Common mode impedance is the impedance measured when stimulating a pair of traces with identical
signals.
8. Examples of a common mode (CM) filter (choke) part numbers are: Coilcraft 0603USB-251ML and
Coilcraft 0805USB-421ML.
9. Common mode chokes can distort signal quality. The signal distortion increases as common mode
impedance increase. Different common mode impedance may be needed to pass EMI testing. The
suggested common mode choke parts were proven by laboratory testing to retain an acceptable signal
waveform.
Copyright © 2017 Marvell
August 30, 2017
Universal Serial Bus (USB) 2.0 Interface
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Document Classification: Public
Interface Signals Layout Guidelines
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Section 2,
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