Ground Planes Under A Discrete Magnetics Module; Ground Plane Separation - Intel 82541PI Design Manual

82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
4.4.7

Ground Planes Under a Discrete Magnetics Module

The magnetics module's chassis or output ground (secondary side of transformer) should be
separated from the digital or input ground (primary side) by a physical separation of 100 mils
minimum. Splitting the ground planes beneath the transformer minimizes noise coupling between
the primary and secondary sides of the transformer and between the adjacent coils in the magnetics.
This arrangement also improves the common mode choke functionality of magnetics module.
Figure 8 illustrates the split plane layout for a discrete magnetics module. Capacitors are used to
interconnect chassis ground and signal ground.
Figure 8. Ground Plane Separation
Figure 9 shows the preferred method for implementing a ground split under an integrated
magnetics module/RJ-45 connector without USB. The capacitor stuffing options (C1-C6) are used
to reduce/filter high frequency emissions. The value(s) of the capacitor stuffing options may be
different for each PCB. Experiments need to be performed to determine which value(s) provide
best EMI performance. Integrated RJ-45 discrete magnetics with USB usually do not have split
planes under them.
34
0.10 Inches Minimum Spacing
Magnetics Module
Void or Separate
Separate Chassis Ground Plane
Ground Plane
Application Note (AP-468)