I found a white paper  from Acromag which discusses how to read differential signal using Acromag units. The document categorically says that differential signals are always supposed to be transmitted in three wires. I provides the two options of either using the RTN to connect to the signal ground (as done in Attachment 3) or locally place 10k-100k resistors between return and IN+ and IN- both (Attachment 2).

I have provided possible scenarios for these.

Using two wires to carry differential signal (Attachment 1):

• I assume this is our preferential way to connect.
• We can also assume all single-ended inputs as differential and do a signal condition agnostic wiring.
• Attachment 3 show what were the results for different values of resistors when a 2Hz 0.5V amplitude signal from SR785 which as converted to differential signal using D1900068 was measured by acromag.
• The connection to RTN is symmetrical for both inputs.

Using three wires to carry differential signal (Attachment 2):

• This is recommended method by the document in which it asks to carry the GND from signal source and connect it to RTN.
• If we use this, we'll have to be very cautious on what GND has been shorted through the acromag RTN terminals.
• This would probably create a lot of opportunities for ground loops to form.

Using an acromag card without making any connection with RTN is basically not allowed as per this document.