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?