Data Sheet

Ethernet Controller I210 —Design Considerations
826
1. Lack of symmetry between the two traces within a differential pair. Asymmetry can create common-
mode noise and distort the waveforms. For each component and/or via that one trace encounters,
the other trace should encounter the same component or a via at the same distance from the
Ethernet silicon.
2. Unequal length of the two traces within a differential pair. Inequalities create common-mode noise
and will distort the transmit or receive waveforms.
3. Excessive distance between the Ethernet silicon and the magnetics. Long traces on FR4 fiberglass
epoxy substrate attenuates the analog signals. In addition, any impedance mismatch in the traces
will be aggravated if they are longer than the four inch guideline.
4. Routing any other trace parallel to and close to one of the differential traces. Crosstalk getting onto
the receive channel causes degraded long cable BER. Crosstalk getting onto the transmit channel
can cause excessive EMI emissions and can cause poor transmit BER on long cables. At a minimum,
other signals should be kept 0.3 inches from the differential traces.
5. Routing one pair of MDI differential traces too close to another pair of differential traces. After
exiting the Ethernet silicon, the spacing between the trace pairs should be kept about 6 times the
dielectric height for stripline and 7 times the dielectric height for microstrip. Refer to the
appropriate design layout checklist for more details. The only possible exceptions are in the
vicinities where the traces enter or exit the magnetics, the RJ-45 connector, and the Ethernet
silicon.
6. Use of a low-quality magnetics module.
7. Re-use of an out-of-date physical layer schematic in a Ethernet silicon design. The terminations and
decoupling can be different from one PHY to another.