Differential impedance is defined as the characteristic impedance between the two lines when the line pair is driven differentially. The microstrip is a very simple yet useful way to create a transmission line with a PCB. Here is an online Microstrip PCB Differential Impedance Calculator to calculate the differential impedance of microstrip using Printed Circuit Board based on the values of characteristic impedance, height of trace, and space between traces.
An electrical transmission line with impedance of 25 Ohm, height of trace as 12 cm, space between traces of 25 cm.
d = 2 x z x (1 - 0.48 x e((-0.96) x (s / h)))
= 2 x 25 x (1 - 0.48 x e((-0.96) x (25 / 12)))
= 50 x 0.48 x 2
= 46.752 Ohm