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82575 Ethernet Controller Design Guide
40
Minimizing the amount of space needed for the Ethernet LAN interface is important
because other interfaces will compete for physical space on a motherboard near the
connector. The Ethernet LAN circuits need to be as close as possible to the connector.
Figure 12. General Placement Distances for 1000 BASE-T Designs
Figure 5 shows some basic placement distance guidelines. The figure shows two
differential pairs, but can be generalized for a Gigabit system with four analog pairs.
The ideal placement for the Ethernet silicon would be approximately one inch behind
the magnetics module.
While it is generally a good idea to minimize lengths and distances, this figure also
illustrates the need to keep the LAN silicon away from the edge of the board and the
magnetics module for best EMI performance.
The following figures illustrate a reference layout for discrete and integrated magnetics.
LAN
Silicon
Integrated
RJ-45
w/LAN
Magnetics
Keep LAN silicon 1" - 4" from LAN connector.
Keep 50 mil minimum distance betewen TX and
RX traces (100 mils is preferred).
Keep silicon traces at least 1" from edge of
PB (2" is preferred).
NOTE: This figure represents a 10/100 diagram. Use the same design considerations for the two differential
pairs not shown for gigabit implementations.