82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
Application Note (AP-468)
27
•
Layer 3 is used for power planes.
•
Layer 4 is a signal layer.
This board stackup configuration can be adjusted to conform to specific design rules.
4.2
Critical Dimensions for Discrete Magnetics Module and RJ-
45
There are four critical dimensions that must be considered during the layout phase of an
82541PI(ER) and 82562GZ(GX) LOM implementation. These dimensions are identified in
as A, B, C, and D.
Figure 5. Critical Dimensions for Component Placement (Discrete Magnetics Solution)
4.2.1
Distance A: Magnetics to RJ45 (Priority 1)
The distance labeled “A” in
should be given the highest priority in board layout. The
distance between the magnetics module and the RJ45 connector should be kept to less than one
inch of separation. The following trace characteristics are important and should be observed:
•
Differential Impedance. The differential impedance should be 100
Ω
. The single ended trace
impedance will be approximately 50
Ω
; however, the differential impedance can also be
affected by the spacing between the traces.
•
Trace Symmetry. Differential pairs should be routed with consistent separation and with
exactly the same lengths and physical dimensions (for example, width).
Caution:
Asymmetrical and unequal length traces in the differential pairs contribute to common mode noise,
which can degrade the receive circuit’s performance and contribute to radiated emissions from the
transmit circuit.
If an Ethernet controller must be placed further than two inches from the RJ45 connector, distance
“B” can be sacrificed. Keeping distance “A” as short as possible should be a priority.
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