What is the typical output signal of loop-powered transmitters?

What is the typical output signal of loop-powered transmitters?

• A. 0-10 VDC output.
• B. Ground-referenced frequency shift.
• C. 4-20 mA DC.
• D. Binary on/off digital signal.

Answer: (C)

Loop-powered transmitters deliver the measurement as a current in a two-wire loop, typically 4–20 mA. The device is powered from the loop and adjusts the current to reflect the process variable: 4 mA corresponds to the low end and 20 mA to the high end. This current signal is ideal for long runs because it’s largely unaffected by cable resistance or supply variations. In practice, a resistor in the receiving instrument converts that current back to a voltage for reading (for example, with 250 ohms, 4 mA yields 1 V and 20 mA yields 5 V). Digital communication (like HART) can ride on top of the 4–20 mA signal, but the primary output representing the measurement remains a 4–20 mA current. Other options describe voltage output, a frequency-based signal, or a discrete on/off signal, none of which are the standard way loop-powered transmitters convey the analog measurement.

Loop-powered transmitters deliver the measurement as a current in a two-wire loop, typically 4–20 mA. The device is powered from the loop and adjusts the current to reflect the process variable: 4 mA corresponds to the low end and 20 mA to the high end. This current signal is ideal for long runs because it’s largely unaffected by cable resistance or supply variations. In practice, a resistor in the receiving instrument converts that current back to a voltage for reading (for example, with 250 ohms, 4 mA yields 1 V and 20 mA yields 5 V). Digital communication (like HART) can ride on top of the 4–20 mA signal, but the primary output representing the measurement remains a 4–20 mA current. Other options describe voltage output, a frequency-based signal, or a discrete on/off signal, none of which are the standard way loop-powered transmitters convey the analog measurement.