LXT970A — Dual-Speed Fast Ethernet Transceiver
3.3
Design Recommendations
The LXT970A is designed in accordance with IEEE requirements and provides outstanding
receive Bit Error Ratio (BER) and long-line-length performance. Lab tests show that the
LXT970A performs well beyond the required distance of 100 meters. Ensuring maximum
performance from the LXT970A requires attention to detail and good design practices. Refer to
the LXT970 Design and Layout Guide for detailed design and layout information.
3.3.1
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
µ
F 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 LXT970A and the RJ-45 connectors at the edge of
the board.
edge of the board. Use this area for chassis ground, or leave it void.
•
Do not extend any circuit power and ground plane past the center of the magnetics or to the
3.3.2
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 by following good general design guidelines. Intel also
recommends filtering between the power supply and the analog VCC pins of the LXT970A.
Filtering has two benefits. First, it keeps digital switching noise out of the analog circuitry inside
the LXT970A, which helps line performance. Second, if the VCC planes are laid out correctly, it
keeps digital switching noise away from external connectors, reducing EMI problems.
42
Datasheet
INTEL [ INTEL CORPORATION ]
