The traditional method mentioned by Erik is to let the sender (sensor) device have its supply float with the 4-20mA current loop. This refers to the device which modulates the 4-20mA current in the loop and not the device that supplies the loop current sourcing or the device which detects the reading corresponding to the modulated loop current. (Note that the loop current source and the receiver (detector) device may be one and the same device).

There is a separate question as to whether the receiver/detector device requires isolation between its 4-20mA loop circuitry and the other parts of its surrounding system. Determination of this requirement has to be made carefully.

Michael Karas