82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
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Application Note (AP-468)
Observe the following suggestions to help optimize board performance. Note that some
suggestions are specific to a 4.3 mil stackup for the 82541ER:
•
Maintain constant symmetry and spacing between the traces within a differential pair.
•
Keep the signal trace lengths within a differential pair equal to each other.
— For the 82541PI, do not use serpentines to try to match trace lengths in the differential
pair. Serpentines cause impedance variations causing signal reflections, which can be a
source of signal distortion. Try to keep the length difference of the differential pair less
than 50 mil (~15 pS). Always go straight to the required via or pad.
•
Keep the total length of each differential pair under four inches. Designs with differential
traces longer than five inches are much more likely to have:
— Degraded receive Bit Error Rate (BER) performance
— IEEE PHY conformance failures
— Excessive EMI radiation
•
For the 82562GZ(GX), keep at least 5 times the distance to the reference plane between Tx
and Rx differential pairs.
•
For the 82541PI(ER), keep at least 5 times the distance to the reference plane between all four
Tx and Rx differential pairs.
•
For all Ethernet controllers, do not route any other signal traces parallel to the differential pairs
closer than 5 times the distance to the reference plane.
•
For the 82562GZ(GX) and the 82541ER, keep maximum separation within differential pairs to
seven mils.
•
For the 82541PI, separate traces within a differential pair as small as possible down to five to
eight mils. Close separation of the traces allow them to couple well to each other.
•
For high-speed signals, the number of corners and vias should be kept to a minimum. If a 90°
bend is required, it is recommended to use two 45° bends instead. See
Figure 7. Corner Trace Routing
•
For all Ethernet controllers, traces should be routed away from board edges by a distance
greater than the trace height above the ground plane. This allows the field around the trace to
couple more easily to the ground plane rather than to adjacent wires or boards.
•
Do not route traces and vias under crystals or oscillators. This prevents coupling to or from the
clock. And as a general rule, place traces from clocks and drives at a minimum distance from
apertures by a distance that is greater than the largest aperture dimension.
45°
45°
