Differential Microstrip Impedance Calculator

Where,

Z₀ = Single Ended Impedance

Z_d = Differential Impedance

d = Trace Separation

w = Trace Width

t = Trace Thickness 

h = Dielectric Thickness

ε_r = Relative Dielectric Constant

Example,

If w = 10 mil, d = 63 mil, t = 1.2 mil, h = 63 mil and ε_r = 4 then

Z₀ = 138.88 Ω and Z_d = 226.71Ω

Differential Microstrip Impedance

An edge coupled 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 below 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,h, t and w can be ignored as long as they have the same units (mils, mm, inches).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. If you have already calculated the single ended impedance Zo, we have developed another calculator that can be used to calculate the differential impedance using Zo.