Pcb Track Impedance; Figure 4-1 Track Impedance - ARM DS-5 Manual

4.2

PCB track impedance

You can calculate the PCB track impedance using the microstrip impedance formula.
Impedance
where:
h
w
t
E
r
This PCB track impedance formula applies only to microstrips (track on outer layer over a ground
plane).
The dimensions
As an example, the following track (in microstrip form) has an impedance of 51.96Ω:
h
w
t
E
r
As the track width increases, the impedance decreases.
Related concepts
2.12 Series termination on page 2-52.
4.1 Overview of high-speed design on page 4-57.
4.4 Probe modeling on page 4-60.
Related references
4.3 Signal requirements on page 4-59.
ARM 100956_0527_00_en
87
=
(E r + 1.41 )
Height above ground plane (inches)
Trace width (inches), and
Trace thickness (inches)
Relative permittivity of processor/prepreg, and
, , and are shown in the following figure.
h w t
0.005 inch height above ground
0.007 inch width track
0.0014 inch thickness (1 oz. finished weight)
4.5 (FR4 laminate)
Note
Copyright © 2010–2012, 2015–2017 ARM Limited or its affiliates. All rights reserved.
4 Target Board Design for Tracing with ARM DSTREAM
5.98h
In Ω
(0.81w + t)
0.1 < w/h < 2
1 < E
Non-Confidential
4.2 PCB track impedance
< 15
r
w
h
Ground plane

Figure 4-1 Track impedance

t
4-58