There are two types of field instruments available in this world according to the condition of how they send the signal, active and passive instruments. Planning and Selecting the proper field instrument will affect the project schedule and the smoothness of construction execution.
Mistakes in understanding the field instrument type will result in the signal cannot be read by the controller.
Active instruments are devices powered by external power (outside of the controller) and they already have voltage on their output. For analog 4-20 mA signal, the devices send the signal with voltage, they can’t be connected directly to the controller’s I/O modules that are active too or have a different voltage reference.
To connect the active instruments to the PLC/ DCS/ Controller’s I/O, they need galvanic signal isolator (signal splitter/ analog signal conditioning). The galvanic signal isolator can be found from brands like weidmuller, pepperl fuchs, levtech etc.
The examples of the active instruments are commonly dew point sensor/ transmitter, flame and gas detectors, coriolis flow transmitter. although some of them may come in a passive or selectable mode according to the manufacturer.
They are commonly loop-powered but some are externally powered. Common instruments like temperature, flow, pressure transmitters are passive and they are loop powered which means to power up the transmitter they don’t need dedicated external power but power already from cable loop that is also used as the signal circuit, of course you could directly connect them to controller’s I/O module.