Single ended inputs are notorius for having problems with ground loop/common mode problems.
Ground loops are most evident with powered, active analog outputs from (4 wire or 3 wire) devices like analyzers because the ground that the AI references is different than the ground the AI references and that difference generates a ground current.
2 wire, loop powered transmitters tend to be isolated and have fewer problems if the loop power supply is on the same circuit as the PLC power.
There are a couple mud-slinging/shouting threads on this forum on the issue of grounding the field instrument DC power supply - whole separate issue, but when if one side's grounded, there is greater potential (pun intended) for ground loops.
Low grade ground loops manifest themselves as offsets in the signal, serious ground loops will drive the signal off-scale and bad boy ground loops will fry your input (mmmm, the smell of burned electronics in the morning).
On single ended cards, one might connect a couple analog inputs that work fine, then connect a 3rd analog input to find that none of the signals are on scale. The ground loop on the 3rd signal has forced all the signals offscale.
In a classroom lab, you stand a fairly good chance of getting multiple inputs to function OK on a single ended card because you're not running 1000m of twisted pair to bring in an analyzer signal from bldg 51.
Users who 'save money' on single ended cards in the real world bear the consequences of their own folly when they have to spend $300-$400 for a loop isolator for each channel of their 'affordable' single ended analog input. Hey, it's cost of doing business.
Isolation pays for itself in the real world.
