Some background: I'm designing an industrial control board with isolated analog and digital IO. The hope is for it to be pretty foolproof, such that you could connect any 4-20 mA or 0-5V sensor with little configuration (i.e. a DIP to switch between current and voltage measurements) and have it work without having to worry too much about grounding specifics. For example, I'd want to a technician to be able to connect both ground-referenced and non ground-referenced sensors without having to think about how the grounding is configured. Some of these sensors may be powered by a separate mains supply, while others may be powered off the board's (isolated) 0-5V supply.
To me, it seems like the way to do this is to have a floating ADC that digitizes the analog signals then sends them to the earth-referenced board through opto-isolators, rather than deal with analog isolation.
My question is, to safely do this, will each analog input require a separate supply? Thinking about it, I think if I could guarantee that all sensor signals would be floating and not referenced to their case ground it would be fine, but I can't. As in the schematic below (sensors are modeled as voltage sources), if two sensors on the same floating supply have their analog ground connected to the case ground, you create a ground loop, conceivably the two sensors at different locations could have different ground potentials/ An example of this would be two pressure transducers with case grounds in different water tanks.
If so, what's a good cheap way to get several floating supplies? Or is there a better way to do what I'm thinking?