Data Sheet

Ethernet Controller I210 —Design Considerations
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12.5.6.8 Trace Length and Symmetry for 1000 BASE-T Designs
As indicated earlier, the overall length of differential pairs should be less than four inches measured
from the Ethernet device to the magnetics.
The differential traces (within each pair) should be equal in total length to within 50 mils (1.25 mm)
and as symmetrical as possible. Asymmetrical and unequal length traces in the differential pairs
contribute to common mode noise. If a choice has to be made between matching lengths and fixing
symmetry, more emphasis should be placed on fixing symmetry. Common mode noise can degrade the
receive circuit’s performance and contribute to radiated emissions.
12.5.6.9 Magnetics Center Tap
The I210 includes a voltage mode driver so it doesn’t require an analog powered center tap. The
decoupling capacitors for the central tap pins should be placed as close as possible to the magnetic
component. This improves EMI compliance.
12.5.6.10 Impedance Discontinuities
Impedance discontinuities cause unwanted signal reflections. Minimize vias (signal through holes) and
other transmission line irregularities. If vias must be used, a reasonable budget is two per differential
trace. Unused pads and stub traces should also be avoided.
12.5.6.11 Reducing Circuit Inductance
Traces should be routed over a continuous reference plane with no interruptions. If there are vacant
areas on a reference or power plane, the signal conductors should not cross the vacant area. This
causes impedance mismatches and associated radiated noise levels. Noisy logic grounds should be
separated from analog signal grounds to reduce coupling. Noisy logic grounds can sometimes affect
sensitive DC subsystems such as analog to digital conversion, operational amplifiers, etc. All ground
vias should be connected to every ground plane; and similarly, every power via, to all power planes at
equal potential. This helps reduce circuit inductance. Another recommendation is to physically locate
grounds to minimize the loop area between a signal path and its return path. Rise and fall times should
be as slow as possible. Because signals with fast rise and fall times contain many high frequency
harmonics, which can radiate significantly. The most sensitive signal returns closest to the chassis
ground should be connected together. This will result in a smaller loop area and reduce the likelihood of
crosstalk. The effect of different configurations on the amount of crosstalk can be studied using
electronics modeling software.
12.5.6.12 Signal Isolation
To maintain best signal integrity, keep digital signals far away from the analog traces. A good rule of
thumb is no digital signal should be within 300 mils (7.5 mm) of the differential pairs. If digital signals
on other board layers cannot be separated by a ground plane, they should be routed perpendicular to
the differential pairs. If there is another LAN controller on the board, take care to keep the differential
pairs from that circuit away.
Some rules to follow for signal isolation:
• Separate and group signals by function on separate layers if possible. Keep a minimum distance
between differential pairs more than seven times the dielectric thickness away from each other and
other traces, including NVM traces and parallel digital traces.