PDP83TG720SWCSTQ1 datasheet(187/199 Pages) TI

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10 Layout

10.1 Layout Guidelines

10.1.1 Signal Traces

PCB traces are lossy and long traces can degrade signal quality. Traces should be kept short as possible.

Unless mentioned otherwise, all signal traces should be 50-Ω, single-ended impedance. Differential traces

should be 50-Ω single-ended and 100-Ω differential. Take care to ensure impedance is controlled throughout.

Impedance discontinuities will cause reflections leading to emissions and signal integrity issues. Stubs should be

avoided on all signal traces, especially differential signal pairs.

Figure 10-1. Differential Signal Trace Routing

Within the differential pairs, trace lengths should be run parallel to each other and matched in length. Matched

lengths minimize delay differences, avoiding an increase in common mode noise and emissions. Length

matching is also important for MAC interface connections. All transmit signal traces should be length matched to

each other and all receive signal traces should be length matched to each other.

Ideally, there should be no crossover or vias on signal path traces. Vias present impedance discontinuities and

should be minimized when possible. Route trace pairs on the same layer. Signals on different layers should not

cross each other without at least one return path plane between them. Differential pairs should always have a

constant coupling distance between them. For convenience and efficiency, TI recommends routing critical

signals first (that is, MDI differential pairs, reference clock, and MAC IF traces).

10.1.2 Return Path

A general best practice is to have a solid return path beneath all signal traces. This return path can be a

continuous ground or DC power plane. Reducing the width of the return path can potentially affect the

impedance of the signal trace. This effect is more prominent when the width of the return path is comparable to

the width of the signal trace. Breaks in return path between the signal traces should be avoided at all cost. A

signal crossing a split plane may cause unpredictable return path currents and could impact signal quality and

result in emissions issues.

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DP83TG720S-Q1

SNLS604 – SEPTEMBER 2020

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PDP83TG720SWCSTQ1 Datasheet(PDF) 187 Page - Texas Instruments