Part Number: TRF1208B
Hello,
First, I am fully aware that the TRF1208B is an AC coupled fully differential amplifier (FDA) and not a DC coupled FDA.
However, my application requires me to understand how the input and output common modes (CMs) of this FDA work.
I have measured the input and output CMs on a PCB with a TR1208B and 4 SMA connectors soldered. The PCB is connected to a 4-Port vector network analyser (VNA). Input and output CMs are measured with a Hi-Z voltmeter through the VNA internal bias-Tees, see figure below.
I have varied VCC from 3.2V to 3.4V according to the datasheet recommendation and measured the input and output CMs. 
Below are my questions:
Within the recommended supply voltage range, it looks like the output CM reference is about VCC-1.1V.
1) Is it reproducible from part-to-to part and what could be the typical variation of this VCC-1.1V reference ?
When simulating and comparing an ideal FDA with the TRF1208B, one can find that the feedback resistance is about 120 Ohm.
2) What is a typical variation of this resistance ? (+/- 10% ?)
The input CM is below the output CM by about 0.57V. Input bias seems to be fed by the outputs through the feedback resistances.
3) What is a typical variation of this 0.57V voltage shift ?
In advance, thank you for your feedback.
Best regards,
Nicolas