Intel Xeon Processor and Intel E7500/E7501Chipset Compatible Platform Design Guide

Intel

Xeon™ Processor and Intel

E7500/E7501 Chipset Compatible Platform Design Guide 169

I/O Controller Hub 3 (Intel

ICH3-S)

9.7.5 Terminating Unused Connections

In Ethernet designs, it is common practice to terminate unused connections on the RJ45 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, and couples capacitively to the ground plane creating the required 1500 pF

of capacitance. The signals can be routed through 75

Ω resistors to the plane. Stray energy on

unused pins is then carried to the plane.

Termination Plane Capacitance

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 (TDP/TDN and RDP/RDN) from the

unused pairs of the RJ45. Pads may be placed for an additional capacitance to chassis ground,

which may be required if the termplane 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 9-19. Termination Plane

N/C

RJ45

Magnetics module

RDP

RDN

TDP

TDN

Termination plane

Additional capacitance that may be

required for EFT testing