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PCB Differential Impedance Calculator
Note
Equations used are based on the IPC-D-317A document from the IPC organization
(
www.ipc.org
© National Instruments Corporation
Dual Stripline Equations
The equations used to perform the dual stripline calculations are:
Z0 = 30*( ln(8*H/(0.67*3.1415926*(0.8*W+T)))+
ln(8*(H+C)/(0.67*3.1415926*(0.8*W+T))))/sqrt(Er)
Tpd = 84.66667*sqrt(Er)
C0 = Tpd/Z0
L0 = C0*Z0*Z0
To control reflections on high-speed PCBs, it is necessary to make the
traces appear as if they are transmission lines. This is done by calculating
the characteristic impedance of the trace (Zo) and then terminating it with
its characteristic impedance. This makes the trace appear like an infinitely
long transmission line, and it will therefore have no reflections, even
though in reality it has a finite length. (What actually occurs is that all of
the energy that travels down the trace is absorbed, and there is no energy
left to reflect back.) Once you have calculated Zo, you can use it to design
the trace's termination.
If two traces in a differential pair are placed closely together, the
differential impedance (Zdiff) of the pair must be calculated for proper
trace termination. (This is the Differential Impedance Rule.)
There are a number of methods used to terminate transmission lines, for
example, series termination, diode termination, which are beyond the scope
of this manual. We recommend that you refer to any number of available
texts on the subject.
).
The PCB Differential Impedance Calculator performs calculations for
two traces that carry signals that are exactly equal and opposite (a
differential pair).
6-7
Chapter 6
PCB Calculators
NI Ultiboard User Manual
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