10 Layout

10.1 Layout Guidelines

10.1.1 Layout Information

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 and therefore some traces must

turn corners. Figure 10-1 shows progressively better techniques of rounding corners. Only the last example

(BEST) maintains constant trace width and minimizes reflections.

WORST

BETTER

BEST

1W min.

W

Figure 10-1. 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.

10.1.2

Figure 10-2 illustrates an example of a PCB layout with the TMUX1208-Q1. 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.

10.2 Layout Example

A0

EN

N.C.

S1

S2

S3

A1

A2

GND

S5

S6

S4

Via to

grou nd plane

TMUX120 8-Q1

C

S8

D

S7

Wide (low inductance)

trace for power

Via to

grou nd plane

Figure 10-2. TMUX1208-Q1 Layout Example

TMUX1208-Q1

SCDS415A – AUGUST 2019 – REVISED JULY 2020

www.ti.com

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