2-wire 4-20mA transmitters are loop powered, and the sensor and its associated circuitry are supplied by the transmitter. Is it possible to use a sensor or microcontroller that uses a separate supply as the input to my 2-wire transmitter?
Yes, you can. The 2-wire transmitter is meant to be loop powered and you cannot connect the ground of another supply to the IRET of the transmitter as seen in the diagram below.
If you wish to use a sensor with its own supply and ground, you need to isolate the input to the transmitter. You can use a digital isolator such as the ISO7310C or ISO7340C, or an isolation amplifier.
The attached article goes into more detail on 2-wire transmitters that use externally powered input sources.
2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources
If possible, you can switch to a locally powered 3 or 4-wire transmitter that shares a supply with the sensor. This would remove the need for input isolation.