An edge couple differential symmetric stripline transmission line is constructed with two traces referenced to the same reference planes above and below the traces. There is a dielectric material between them. There is also some coupling between the lines. This coupling is one of the features of differential traces. Usually it is good practice to match differential trace length and to keep the distances between the traces consistent. Also avoid placing vias and other structures between these traces.

Differential Impedance Definitions

Differential Impedance: The impedance measured between the two lines when they are driven with opposite polarity signals. Zdiff is equal to twice the value of Zodd

Odd Impedance: The impedance measured when testing only one of the differential traces referenced to the ground plane. The differential signals need to be driven with opposite polarity signals. Zodd is equal to half of the value of Zdiff

Common Impedance: The impedance measured between the two lines when they are driven with the same signal. Zcommon is half the value of Zeven

Even Impedance: The impedance measured when testing only one of the differential traces referenced to the ground plane. The differential signals need to be driven with the same identical signal. Zeven is twice the value of Zcommon

Impedance calculation

Edge coupled microstrip impedances (Z_0,odd, Z_0,even, Z_0,diff and Z_0,common) shall be calculated as specified in previous paragraph.

Example 1

Inputs

T = 1.2 mil

B = 63 mil

W = 10 mil

S = 63 mil

ε_r = 4

Output

Z_odd = 78.4 Ω

Z_even = 78.3 Ω

Z_diff = 156 Ω

Z_common = 39.2 Ω

Example 2

Inputs

T = 1.5 mil

B = 70 mil

W = 15 mil

S = 65 mil

ε_r = 4.3

Output

Z_odd = 68.1 Ω

Z_even = 68.0 Ω

Z_diff = 136 Ω

Z_common = 34.0 Ω

Reference

[1] IPC-2221A “Generic Standard on Printed Board Design”