4-20 mA loop powered ADC with display
For a loop-powered 4/20 mA device, 4 mA is the absolute maximum current your device should need to operate correctly (ideally 2 mA max). That’s your first hurdle. Based on this, it sounds to me like the display should be low power tech like an LCD. There used to be plenty of dual slope integrating ADC chips that directly drive an LCD so that’s where I’d look first. Choose one that only needs a low voltage supply like 5 volts or less. They've been around for decades so I'm not sure how available they still are but they'll give you the simplest implementation I reckon.

The easiest (and most appropriate for current loops) line voltage regulation circuit is a zener diode that drops the 5 volts needed to power the circuit. That zener diode is going to waste a little current so you need to find one that doesn’t steal too much of the 4 mA needed to power the chip and LCD. Maybe choose a precision shunt zener because most of them will only waste sub 100 uA. If your circuit doesn't use the rest of the 4 mA, the zener will act as a shunt regulator and this keeps the "loop" noise-free but, just in case, put a 1 uF capacitor across it.

You also need to have a small series resistor so you can measure a voltage proportional to the current flow (the signal). Likely this will require a diff amp between resistor and ADC chip.

You need to establish how much volt drop the system can allow of course. If 5 volts isn’t a problem then use a zener. If, say 8 volts isn’t a problem then you can use a bridge rectifier as well as a zener so that your device will work on both connection polarities. There is still a lot of detail required but I hope this provides some small insight.

You could cheat a bit to get more current by using one of these: -



Note that it needs at least 10 volts across the loop terminals to guarantee working correctly. LT also have this idea: -