CD54HC4051_15 Datasheet, PDF (25/45 Pages) Texas Instruments – High-Speed CMOS Logic Analog Multiplexers and Demultiplexers

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11 Layout
CD54HC4051, CD74HC4051
CD54HCT4051, CD74HCT4051, CD54HC4052, CD74HC4052, CD54HCT4052
CD74HCT4052, CD54HC4053, CD74HC4053, CD54HCT4053, CD74HCT4053
SCHS122K – NOVEMBER 1997 – REVISED SEPTEMBER 2015
11.1 Layout Guidelines
Reflections and matching are closely related to loop antenna theory, but different enough to warrant their own
discussion. When a PCB trace turns a corner at a 90° angle, a reflection can occur. This is primarily due to the
change in width of the trace. At the apex of the turn, the trace width is increased to 1.414 times its width. This
change in width upsets the transmission line characteristics, especially the distributed capacitance and self-
inductance of the trace, thus resulting in the reflection. Not all PCB traces can be straight, so they will have to
turn corners. Figure 22 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 22. Trace Example
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