Fabrizio,
Differential impedance formulas derive from a 2-port analysis:
| V1 | | Z11 Z12 | | I1 |
| | = | | * | |
| V2 | | Z12 Z11 | | I2 |
where V1 and V2 are, respectively, the voltages at the input to
the positive and negative lines of the pair. I1 and I2 are,
likewise, the currents into these lines. For simplicity, I have
assumed a symmetrical layout (Z11 = Z22).
There might be confusion arising from terminology. I assume
"odd mode impedance" is V1/I1 (= V2/I2) when V1 = -V2, and
"even mode impedance" is V1/I1 (= V2/I2) when V1 = V2. Other
definitions might leave factors of two floating around.
By my assumed definitions, Zoo = Z11 - Z12, and Zee = Z11 + Z12.
Your colleague's formula would seem to be correct only if there
were a factor of two difference in the definitions and if Z12 were
negligible. I like your definition (Zdiff = 2 * Zoo).
Mike
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