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PCB track impedance

Use the following formula only for microstrips (track on outer layer over a ground plane):

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where:

h

Height above ground plane (inches)

w

Trace width (inches), and 0.1 < w/h < 2

t

Trace thickness (inches)

Er

Relative permittivity of processor/prepreg, and 1 < Er < 15

The dimensions h, w, and t are shown in the following figure.

Figure 14. Track impedance

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As an example, the following track (in microstrip form) has an impedance of 51.96Ω:

h

0.005 inch height above ground

w

0.007 inch width track

t

0.0014 inch thickness (1 oz. finished weight)

Er

4.5 (FR4 laminate)

Note

As the track width increases, the impedance decreases.

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