LXT970ATC datasheet(42/74 Pages) INTEL

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LXT970A — Dual-Speed Fast Ethernet Transceiver

Datasheet

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

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 LXT970A 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.

3.3.2

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.

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