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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 1: https://www.mouser.de/datasheet/2/1/ts-0875-C-1155517.pdf

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

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The data sheet says this: -

Note: Unbalanced is measured between ground-signal-ground conductors. Balanced is measured between signal conductors within a pair

So, "unbalanced" is fairly unambiguous but, "balanced" appears to me as if they chose a pair and didn't ground any conductors either side. This is why the capacitance stated is 12 pF / foot in "balanced" and 20.8 pF / foot when "unbalanced".

It's also why the inductance per foot is lower in "unbalanced" (two ground conductors will reduce inductance) at 110 nH / foot whilst in "balanced" the inductance is greater i.e. 180 nH / foot.

The maths: -

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