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signals with fast rise and fall times contain many high frequency harmonics that can radiate
significantly. The most sensitive signal returns closest to the chassis ground should be connected
together. This will result in a smaller loop area and reduce the likelihood of crosstalk. The effect of
different configurations on the amount of crosstalk can be studied using electronics modeling software.
10.9.3.5.1. Terminating Unused Connections
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. The method most often
implemented is called the "Bob Smith" Termination. In this 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- resistors to the plane. Stray energy on unused pins is then
carried to the plane.
10.9.3.5.2. Termination Plane Capacitance
Intel recommends that the termination plane capacitance equal a minimum value of 1500 pF. This helps
reduce the amount of crosstalk on the differential pairs (TDP/TDN and RDP/RDN) from the unused
pairs of the RJ-45. 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 80. Termination Plane
TDP
TDN
RDP
RDN
®
®
Intel
852GME, Intel
852GMV and Intel
N/C
Magnetics Module
Termination Plane
®
852PM Chipset Platforms Design Guide
Addition Capacitance that may need to be
added for EFT testing
I/O Subsystem
RJ-45
183
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