82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
26
Application Note (AP-468)
Figure 4. General Placement Distances
While it is generally a good idea to minimize lengths and distances, this figure also illustrates the
need to keep the Ethernet controller away from the edge of the PCB and the magnetics module for
best EMI performance.
4.1.1
Crystals
Crystals should not be placed near I/O ports or PCB edges. Radiation from these devices may be
coupled onto the I/O ports or out of the system chassis. Crystals should also be kept away from the
Ethernet magnetics module to prevent interference. Traces should be referenced to a continuous
low impedance ground plane.
Place the crystal and its load capacitors on the PCBs as close to the LAN device as possible (within
0.75-inch). Keep other potentially noisy traces away from the crystal traces.
The restraining straps of the crystal should be grounded to reduce the possibility of radiation from
the crystal’s case. Crystals should lay flat against the PCB to provide better coupling of
electromagnetic fields to the PCB.
4.1.2
Board Stackup Recommendations
PCBs for these designs typically have four, six, eight, or more layers. Following is a description of
a typical four-layer board stackup:
•
Layer 1 is a signal layer. It can contain the differential analog pairs from the Ethernet device to
the magnetics module.
•
Layer 2 is a signal ground layer. Chassis ground may also be fabricated in Layer 2 under the
connector side of the magnetics module.
Ethernet
Controller
Integrated
RJ-45
w/LAN
Magnetics
Keep 5x the distance to the reference plane between
Tx and Rx differential pairs
Keep Ethernet controller 1 to 4 inches from LAN
connector
Keep Ethernet controller and traces at least 1 inch from
edge of PB (2 inches is preferred)
