Magnetics Module General Power And Ground Plane Considerations; Figure 125. Ground Plane Separation - Intel 855PM Design Manual

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9.9.6.1.3.

Magnetics Module General Power and Ground Plane Considerations

To properly implement the common mode choke functionality of the magnetics module the 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

Figure 125. Ground Plane Separation

Good grounding requires minimizing inductance levels in the interconnections and keeping ground
returns short, signal loop areas small, and power inputs bypassed to signal return, will significantly
reduce EMI radiation.
Some rules to follow that will help reduce circuit inductance in both back planes and motherboards.
Route traces over a continuous plane with no interruptions (don't route over a split plane). If there
are vacant areas on a ground or power plane, avoid routing signals over the vacant area. This will
increase inductance and EMI radiation levels.
Separate noisy digital grounds from analog grounds to reduce coupling. Noisy digital grounds may
affect sensitive DC subsystems.
All ground vias should be connected to every ground plane; and every power via should be
connected to all power planes at equal potential. This helps reduce circuit inductance.
Physically locate grounds between a signal path and its return. This will minimize the loop area.
Avoid fast rise/fall times as much as possible. Signals with fast rise and fall times contain many
high frequency harmonics, which can radiate EMI.
The ground plane beneath the filter/transformer module should be split. The RJ-45 connector side
of the transformer module should have chassis ground beneath it. By splitting ground planes
beneath transformer, noise coupling between the primary and secondary sides of the transformer
and between the adjacent coils in the transformer is minimized. There should not be a power plane
under the magnetics module.
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Intel 855PM Chipset Platform Design Guide
0.10 Inches Minimum Spacing
Magnetics Module
Void or Separate
Separate Chassis Ground Plane
I/O Subsystem
Ground Plane
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