Where,
Zd = Differential Impedance
Z0 = Single Ended Impedance
d = Trace Separation
h = Dielectric Thickness
Example,
If Z0 = 50 Ω, d = 5 mils and h = 1.6 mils then,
Zd = 97.6102 Ω

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Differential Microstrip Impedance

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A differential microstrip transmission line is constructed with two traces on the same reference plane. These lines are placed on a dielectric material of height h and there is also some coupling between the lines. It is good practice to match differential trace length and to keep the distances between the traces consistent.
As can be seen from the formula above when d decreases, while keeping h constant, differential impedance decrease. The differential impedance depends upon the D/H ratio. For this calculation, the units of d and h can be ignored as long as they have the same unit.
Calculating the Differential impedance is a two-step process, first Calculate the single ended impedance Zo and then use this value to calculate the differential impedance. This calculator assumes that you already have the value for Zo

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