I used an Acromag XT1221 in CTN to play around with different wiring and see what works.  Following are my findings:

Referenced Single Ended Source (Attachment 1):

• If the source signal is referenced single ended, i.e. the signal is only on the positive output and the negative side is tied to GND on the source side AND this GND is also shared by the power supply powering the Acromag, then no additional wiring is required.
• The GND common to the power supply and the source is not required to be Earth GND but if done so, it should be at one point only.
• RTN terminal on Acromag can be left floating or tied to IN- terminal.

Floating Single Ended Source (Attachment 2):

• If the source signal is floating single-ended i.e. the signal is only on the positive output and the negative output is a floating GND on the source, the the IN- should be connected to RTN.
• This is the case for handheld calibrators or battery powered devices.
• Note that there is no need to connect GND of power supply to the floating GND on the source.

Differential Source (Attachment 3):

• If the source is differential output i.e. the signal is on both the positive output and the negative output, then connect one of the RTN terminals on Acromag to Earth GND. It has to be Earth GND for this to work.
• Note that you can no longer tie the IN- of different signals to RTN as they are all carrying different negative output from the source.
• Earth GND at RTN gives acromag a stable voltage reference to measure against the signals coming in IN+ and IN-. And the most stable voltage reference is of course Earth GND.

Conclusion:

• We might have a mix of these three types of signals coming to a single Acromag box. In that case, we have to make sure we are not connecting the different IN- to each other (maybe through RTN) as the differential negative inputs carry signal, not a constant voltage value.
• In this case, I think it would be fine to always use differential signal wiring diagram with the RTN  connected to Earth GND.
• There's no difference in software configuration for the two types of inputs, differential or single-ended.
• For cases in which we install the acromag box inside a rack mount chasis with an associated board (example: CTN/2248), it is ok and maybe the best to use the Attachment 1 wiring diagram.

Comments and suggestions are welcome.

Related elog posts:

40m/14841    40m/15134

Edit Tue Jan 26 12:44:19 2021 :

Note that the third wiring diagram mentioned actually does not work. It is an error in judgement. See 40m/15762 for seeing what happens during this.