# What's the cable configuration of a ribbon cable when aiming for a balanced characteristic impedance?

The ribbon cable in reference 1 states a balanced characteristic impedance of 122 Ω. For which configuration of the wires is this value valid? From reference 2, it becomes evident that the characteristic impedance is influenced by the arrangement. Two configurations are discussed there:

G S+ S- G
G S+ G S- G


G is ground, S+ is signal+ and S- is signal-

To my understanding, the latter configuration should result in a balanced characteristic impedance of exactly two times the unbalanced impedance of 73 Ω, thus giving 146 Ω - but the balanced characteristic impedance is stated as 122 ohms. Therefore, I think the first configuration was chosen when the characteristic impedance of the ribbon cable was measured.

Reference 2: http://www.sigcon.com/Pubs/news/3_10.htm

• Unbalanced $$\Z_0 = \sqrt{110/0.0208} = 72.7\space\Omega\$$
• Balanced $$\Z_0 = \sqrt{180/0.012} = 122.5\space\Omega\$$