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Common mode chokes distort full-speed and high-speed signal quality. As the common mode
impedance increases the distortion increases, therefore test the effects of the common mode choke on
full speed and high-speed signal quality. Common mode chokes with a target impedance of 80 Ω to 90 Ω,
at 100 MHz, generally provide adequate noise attenuation.
Finding a common mode choke that meets the designer's needs is a two-step process:
1. Choose a part with the impedance value that provides the required noise attenuation. This is a
function of the electrical and mechanical characteristics of the part chosen and the frequency and
strength of the noise present on the USB traces that should be suppressed.
2. After obtaining a part that gives passing EMI results, the second step is to test the effect this part has
on signal quality. Higher impedance common mode chokes generally have a greater damaging effect
on signal quality, so care must be used when increasing the impedance without doing thorough testing.
Thorough testing means that the signal quality must be checked for low-speed, full-speed, and
highspeed USB operation.
Further common mode choke information can be found on the high-speed USB Platform
Design Guides available at www.usb.org.
2.7.2.4 EMI / ESD Protection
To improve the EMI behavior of the USB interface, a design should include common mode chokes,
which have to be placed as close as possible to the USB connector signal pins.
Common mode chokes can provide required noise attenuation but they also distort the signal quality of
full-speed and high-speed signaling. Therefore, common mode chokes should be chosen carefully to
meet the requirements of the EMI noise filtering while retaining the integrity of the USB signals on the
Carrier Board design.
To protect the USB host interface of the Module from over-voltage caused by electrostatic discharge
(ESD) and electrical fast transients (EFT), low capacitance steering diodes and transient voltage
suppression diodes have to be implemented on the Carrier Board design. In the USB reference
schematics Figure 29 above, this is implemented by using 'SR05 RailClampR' surge rated diode arrays
from Semtech (http://semtech.com).
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