82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
Application Note (AP-468)
33
Some rules to follow for signal isolation:
•
Separate and group signals by function on separate layers if possible. Maintain a gap of 5
times the distance to the reference plane between all differential pairs (Ethernet) and other
nets, but group associated differential pairs together.
•
Isolate I/O signals from high-speed signals to minimize crosstalk, which can increase EMI
emission and susceptibility to EMI from other signals.
•
Avoid routing high-speed LAN traces near other high-frequency signals associated with a
video controller, cache controller, processor, or other similar devices.
4.4.5
Power and Ground Planes
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.
The following guidelines help reduce circuit inductance in motherboards:
•
Route traces over a continuous plane with no interruptions. Do not route over a split power or
split ground plane. If there are vacant areas on a ground or power plane, avoid routing signals
over the vacant area. This increases inductance and EMI radiation levels.
•
Separate noisy digital grounds from analog grounds to reduce coupling. Noisy digital grounds
can affect sensitive DC subsystems.
•
All ground vias should be connected to every ground plane layer; similarly, every power via
should be connected to all equal potential power plane layers. This helps reduce circuit
inductance.
•
Use vias in pairs. Two (or more) small vias have less inductance and are preferable over large
vias. The small vias might also require less board space.
•
Physically locate grounds between a signal path and its return. This minimizes 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.
•
If the magnetics module is a discrete module, the ground plane beneath the magnetics module
should be split. The RJ45 connector side of the transformer module should have chassis
ground beneath it (see
•
Power planes are not recommended as reference (or AC ground) planes for the differential
since most of them are noisy and can contaminate signals.
4.4.6
Traces for Decoupling Capacitors
Traces between decoupling and I/O filter capacitors should be as short and wide as practical. Long
and narrow traces are more inductive and would reduce the intended effect of decoupling
capacitors. Also for similar reasons, traces to I/O signals and signal terminations should be as short
as possible. Vias to the decoupling capacitors should be sufficiently large in diameter to decrease
series inductance.
