Signal Integrity TUTORIAL

Differential Impedance Calculator - Microstrip

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

Following is the formula for differential impedance for a Microstrip.

Z diff = 2* Zo (1 - 0.48 e-0.96D/H)

Zo = 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, Zdiff decrease. It actually depends upon the D/H ratio.

Calculatiing the differential impedance is a two two step process. First Calculate the single ended impedance Zo. In the next step you calculate the differential impedance.

Zo is in Ohms. The Units of H and D does not matter as long as both are same. They should be both in mils or both in meters, for example. After entering values, click on Zdiff button.

Zo Characteristic Impedance :
D edge to edge spacing
H distance to nearest reference plane
  (Differential Impedance) :

In the next page we will see the formula for stripline differential impedance calculator.