This automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.
To change a 4–20 mA analog signal from a transmitter to the signal required by a digital controller, a(n) _____ must be between the transmitter and controller in the measurement loop.
a) I/P transducer
b) P/I transducer
c) DP transmitter
d) A/D converter
e) none of the above
Click Here to Reveal the Answer
An “I/P transducer” is used to convert an analog current (I) signal to a pneumatic (P) signal, as for actuation of final control elements.
A “P/I transducer” is used to convert a pneumatic signal (P) to an analog current (I) signal, as for a pneumatic transmitter in a programmable logic controller loop.
A “DP transmitter” is a differential pressure transmitter, which can output a pneumatic, an analog, or a digital signal, depending on the model of transmitter used.
The correct answer is D, “A/D converter.” A digital controller requires a digital signal as its input. A 4–20 mA transmitter outputs an analog signal. Therefore, a device to convert an analog (A) signal to a digital (D) is required. This class of device is referred to as an “A/D converter.”
Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition