Typical differential input impedance for IC

The typical impedance used in RF is 50 Ohm and if I build an RFIC with a "RFIN" pin I can expect its source impedance to be 50 Ohm.

However, how does the situation look like in a differential setup, when I have, say RFIN+ and RFIN-?

50 Ohm Single Ended would be 25 Ohm differential.

Does that mean that the source impedance of the RFIN+ pin can be assumed 25 Ohm or would it still be 50 Ohm? (which would mean that externally it would sum up to 100 Ohm).

I know this depends on the application but I am interested in the most common case. Take for example the AD9371 ("Integrated Dual RF Transceiver With Obs Path"). It has differential RF input, RX1+ and RX1-. I would assume that the signal source has 50 Ohm single ended and after the balun this becomes 25 Ohm so that each RX1+/RX1- "sees" 25 Ohm source.

Now the datasheet is more confusing to me. It states: "Input impedance: 200 Ohm Differential". What does that mean? This would imply that the input to each RX1+/RX1- pin is 100 Ohm? And to achieve matching, the source impedance would have to be 200 Ohm, 4x than the common 50 Ohm. Why is this the case?

You have to understand mode decomposition, even(=differential) and odd(=single-ended).

If you consider only even mode, signal source impedance should be 25ohm as singled-ended, this is equivalent to floating source impedence of 50ohm as far as concering even mode.

These concepts are linear, so it is not difficult to understand.


Generally it means impedance of completely differential mode.

It is correct, if you drive completely diffrentially.

I can not understand what you mean at all.

See https://www.edaboard.com/thread352410.html#2

You would prefer a specific differential input impedance, e.g. most common 100 ohm, if impedance matching to a differential source of same impedance is wanted, e.g. to a differential cable. AD9371 receiver port has a rather high noise figure so that the performance doesn't depend on lossless matching. Instead it will be typically used with an external LNA. Impedance matching, if required at all, can be e.g. achieved with load resistors.

The chip has differential RF in- and output primarily for reduced crosstalk. To utilize the common mode rejection in a design, active devices with differential interface or balun transformer will be used. Impedance matching can be performed on the way.