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# ideal balun input/output impedance

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#### exp

##### Full Member level 1
I am a bit confused about the input/output impedance of the ideal_balun of analogLib.

What resistance at the differential outputs is required so that the common mode input sees 50 Ohm?

I use a port with Vin=50mV and 50 Ohm. When I connect 50 Ohm to the port and look at the amplitude it is exactly what expected: 50mV.

However, when I connect the ideal balun to the port and then put the 50 Ohm to "n" and "p", the voltage is higher, 66mV.

I tried different factors of 1/2, 2, sqrt(2), 1/sqrt(2) etc.

Thank you.

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.

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.

The idealBalun of analogLib is implemented with ideal transformers with N=2 at the output I think.

So if I want to see 50 Ohm at the DM input I would expect to connect 50*sqrt(2) or 50/sqrt(2) to "+" or "-". But this is not the case ...

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