LXT974/LXT975 — Fast Ethernet 10/100 Quad Transceivers
3.0
Application Information
3.1
Design Recommendations
The LXT974/975 is designed to comply with IEEE requirements and to provide outstanding
receive Bit Error Rate (BER) and long-line-length performance. Lab testing has shown that the
LXT974/975 can perform well beyond the required distance of 100 meters. To achieve maximum
performance from the LXT974/975, attention to detail and good design practices are required.
Refer to the LXT974/975 Design and Layout Guide for detailed design and layout information.
3.1.1
General Design Guidelines
Adherence to generally accepted design practices is essential to minimize noise levels on power
and ground planes. Up to 50 mV of noise is considered acceptable. 50 to 80 mV of noise is
considered marginal. High-frequency switching noise can be reduced, and its effects can be
eliminated, by following these simple guidelines throughout the design:
• Fill in unused areas of the signal planes with solid copper and attach them with vias to a VCC
or ground plane that is not located adjacent to the signal layer.
• Use ample bulk and decoupling capacitors throughout the design (a value of .01 µF is
recommended for decoupling caps).
• Provide ample power and ground planes.
• Provide termination on all high-speed switching signals and clock lines.
• Provide impedance matching on long traces to prevent reflections.
• Route high-speed signals next to a continuous, unbroken ground plane.
• Filter and shield DC-DC converters, oscillators, etc.
• Do not route any digital signals between the LXT974/975 and the RJ-45 connectors at the edge
of the board.
• Do not extend any circuit power and ground plane past the center of the magnetics or to the
edge of the board. Use this area for chassis ground, or leave it void.
3.1.2
Power Supply Filtering
Power supply ripple and digital switching noise on the VCC plane can cause EMI problems and
degrade line performance. It is generally difficult to predict in advance the performance of any
design, although certain factors greatly increase the risk of having these problems:
• Poorly-regulated or over-burdened power supplies.
• Wide data busses (>32-bits) running at a high clock rate.
• DC-to-DC converters.
Many of these issues can be improved just by following good general design guidelines. In
addition, Intel also recommends filtering between the power supply and the analog VCC pins of the
LXT974/975. Filtering has two benefits. First, it keeps digital switching noise out of the analog
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Datasheet
INTEL [ INTEL ]
