[FAQ] Can I use a 2-wire 4-20mA transmitter with a sensor or microcontroller that uses its own supply?

Yes, you can. The 2-wire transmitter is meant to be loop powered and you cannot connect the ground of another supply to the I_RET 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.