Intel 82547EI, Design Manual

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82562EZ(EX)/82547GI(EI) Dual Footprint Design Guide
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4.1.13 Special Considerations for Non-Integrated Magnetics Modules and
RJ-45 Connectors
It is possible to employ discrete (non-integrated) magnetics modules and RJ-45 connectors. Similar
rules will apply to design and layout. The differential pairs should be routed to be as short and
symmetrical as possible and the overall lengths of the differential pairs (including the width of the
magnetics module) should not exceed approximately four inches.
Additional design and layout steps will be required to add a dedicated board termination plane
parallel to chassis ground, 75
Ω
termination resistors, and a 1500 pF capacitor. This “Bob Smith”
termination scheme is normally contained inside an integrated magnetics module.
In Ethernet designs, it is common practice to terminate unused connections on the RJ-45 connector
and the magnetics module to ground. Depending on overall shielding and grounding design, this
may be done to the chassis ground, signal ground, or a termination plane. Care must be taken when
using various grounding methods to insure that emission requirements are met. In the “Bob Smith”
termination method, a floating termination plane is cut out of a power plane layer. This floating
plane acts as a plate of a capacitor with an adjacent ground plane. The signals can be routed
through 75
Ω
resistors to the plane. Stray energy on unused pins is then carried to the plane.
It is recommended that the termination plane capacitance equal a minimum value of 1500 pF. This
helps reduce the amount of crosstalk on the differential pairs from the unused pairs of the RJ-45
connector. Pads may be placed for an additional capacitance to chassis ground, which may be
required if the termination plane capacitance is not large enough to pass EFT (Electrical Fast
Transient) testing. If a discrete capacitor is used, to meet the EFT requirements it should be rated
for at least 1000 Vac.
Figure 9. Termination Plane Example for 82562EZ(EX) PLC Device and Discrete Magnetics
N/C
RJ-45
Magnetics module
RDP
RDN
TDP
TDN
Termination plane
Additional capacitance that may be
required for EFT testing
LAN_term_plane