SN74CB3Q3125_15 Datasheet, PDF (12/26 Pages) Texas Instruments – Quadruple FET Bus Switch

SN74CB3Q3125
SCDS143C – OCTOBER 2003 – REVISED JUNE 2015
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11 Layout
11.1 Layout Guidelines
Reflections and matching are closely related to the loop antenna theory but are different enough to be discussed
separately from the theory. When a PCB trace turns a corner at a 90° angle, a reflection can occur. A reflection
occurs primarily because of the change of width of the trace. At the apex of the turn, the trace width increases to
1.414 times the width. This increase upsets the transmission-line characteristics, especially the distributed
capacitance and self-inductance of the trace, which results in the reflection. Not all PCB traces can be straight;
therefore, some traces must turn corners. Figure 7 shows progressively better techniques of rounding corners.
Only the last example (BEST) maintains constant trace width and minimizes reflections.
11.2 Layout Example
WORST
BETTER
BEST
2W
1W min.
W
Figure 7. Trace Example
12
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