The Balun is a circuit that is transparent. By which we mean that any impedance on either side is mirrored to the other side.
This Balun below is implemented with two transformers like this 200:50 balun with 4x for the common mode C tap to ground. Where the tapped side is unbalanced and the +/- side is balanced differential.
CM impedance is determined by impedance on port C.
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To test any DUT, match the impedance on both sides, using a 1:1 Balun with 10k in for an SPI chip and 50 out using two baluns.
The Differential impedance is a matter of design application for noise reduction, balance to unbalanced conversion, and power loss.
So DM impedance is never the same for all applications. For example on AC power lines or stepper motor drivers , you want the outside side CM impedance to be high to interference and shunted by caps if required on the inside CM port to attenuate CM noise. Here the CM impedance is not balanced on both sides.
For you application, you want equal DM and CM impedance so you must have 3 resistors on each side , 2 differential and 1 series to match both requirements.