Differemtial Impedance

8.7 Differential Impedance
The differential signals have a differential characteristic impedance. The PCB designer must ensure that its characteristic impedance has the desired value.

Online calculators are available to calculate the differential impedance. The differential impedance of a microstrip can be calculated using the online calculators.

8.8 Microstrip Differential impedance

The differential impedance between the positive and negative trace of the microstrip transmission line is slightly less than twice the impedance of the single ended impedance. The closer the two traces, the smaller is the differential impedance.

Following is the formula for differential impedance for a Microstrip


where,

Z_diff = Differential impedance
Z_o = Single Ended impedance
H = distance to nearest reference plane
D = trace edge to edge spacing

As seen from the formula when D decreases, while keeping H constant, decrease. It actually depends upon the D/H ratio.

Calculating the differential impedance using calculators is a two step process. First we calculate the single ended impedance . In the next step you calculate the differential impedance. The Calculators are not very accurate but they do give you quick answers for initial estimates.


Figure 8.1 – A differential microstrip trace

You can use the online calculator at
http://www.referencedesigner.com/tutorials/si/si_10.php
to calculate the microstrip differential calculator.


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