Via to

GND plane

C

Wide (low inductance)

trace for power

TMUX1237

WORST

BETTER

BEST

1W min.

W

24

TMUX1237

SCDS424A – DECEMBER 2019 – REVISED MARCH 2020

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Copyright © 2019–2020, Texas Instruments Incorporated

11 Layout

11.1 Layout Guidelines

11.1.1 Layout Information

When a PCB trace turns a corner at a 90° angle, a reflection can occur. A reflection primarily occurs because the

width of the trace changes. At the apex of the turn, the trace width increases to 1.414 times its 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 and therefore some traces must

turn corners. Figure 21 shows progressively better techniques of rounding corners. Only the last example (BEST)

maintains constant trace width and minimizes reflections.

Figure 21. Trace Example

Route high-speed signals using a minimum of vias and corners which reduces signal reflections and

impedance changes. When a via must be used, increase the clearance size around it to minimize its

capacitance. Each via introduces discontinuities in the signal’s transmission line and increases the chance of

picking up interference from the other layers of the board. Be careful when designing test points, through-

hole pins are not recommended at high frequencies.

Figure 22 illustrates an example of a PCB layout with the TMUX1237. Some key considerations are:

•

•

Keep the input lines as short as possible.

•

Use a solid ground plane to help reduce electromagnetic interference (EMI) noise pickup.

•

Do not run sensitive analog traces in parallel with digital traces. Avoid crossing digital and analog traces if

possible, and only make perpendicular crossings when necessary.

11.2 Layout Example

Figure 22. TMUX1237 Layout Example