12 Layout

12.1 Layout Guidelines

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 12-1 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 12-1. 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.

Some key considerations are:

• Decouple the supply pins with a 0.1-µF and 1 µF capacitor, placed lowest value capacitor as close to the pin

as possible. Make sure that the capacitor voltage rating is sufficient for the supply voltage.

• 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.

• Using multiple vias in parallel will lower the overall inductance and is beneficial for connection to ground

planes.

12.2 Layout Example

SEL1

D1

S1

VSS

D2

S2

TMUX721x

VDD

SEL2

N.C.

S3

D3

SEL3

GND

S4

D4

SEL4

Wide (low inductance)

trace for power

Wide (low inductance)

trace for power

Via to ground plane

Figure 12-2. TMUX721x Layout Example

TMUX7211, TMUX7212, TMUX7213

SCDS416B – OCTOBER 2020 – REVISED APRIL 2021

www.ti.com

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