DP83867CS datasheet(104/114 Pages) TI1

104 / 114 page

DP83867CS, DP83867IS, DP83867E

SNLS504 – OCTOBER 2015

www.ti.com

12 Layout

12.1 Signal Traces

PCB traces are lossy and long traces can degrade the 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 100Ω differential. Care should be taken that the impedance is constant throughout.

Impedance discontinuities will cause reflections leading to EMI & signal integrity problems. Stubs should be

avoided on all signal traces, especially the differential signal pairs. See Figure 28

Within the differential pairs, the 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 increased EMI.

Length matching is also important on MAC interface. 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 via on the signal paths. Vias present impedance discontinuities and

should be minimized. Route an entire trace pair on a single layer if possible.

Figure 28. Avoiding Stubs in a Differential Signal Pair

Signals on different layers should not cross each other without at least one return path plane between them.

Coupling between traces is also an important factor. Unwanted coupling can cause cross talk problems.

Differential pairs on the other hand, should have a constant coupling distance between them.

For convenience & efficient layout process, start by routing the critical signals first.

12.2 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 width 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 beneath the signal traces should be avoided at all cost. A

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

as well, potentially creating EMI problems. See Figure 29

104

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DP83867CS Datasheet(PDF) 104 Page - Texas Instruments