What is a Two-Wire Transmitter and 4-20mA Current Loop? An Introduction – 8500904

What Makes 4-20mA Signal Transmission so Outstanding?

In two-wire 4-20mA control loops, we use 2-wire transmitters to convert various process signals representing flow, speed, position, level, temperature, pressure, strain, pH, etc., to 4-20mA DC for the purpose of transmitting the signal over some distance with little or no loss of signal.

This paper reviews the operation of this transmission standard and its advantages, in particular as it relates to two-wire transmitters and the associated 4-20mA current loop.

Advantages of Using a Current Loop for Signal Transmission

• The greatest advantage of using a current loop for signal transmission is the current loop’s low sensitivity to electrical noise. This is very important for long distance transmission in harsh industrial environments.
• Another advantage to the 4-20mA current loop is that it is essentially lossless with respect to the transmission media (wire) and the interconnections (connectors).
• The 4mA “Zero-Offset”, “Live Zero”, or “Positive-Zero” is Failsafe. The use of 4mA as the starting point for our transmitted signal is useful in troubleshooting, as signal integrity is verified with 0% of input and output signal.

Acromag offers several types of 2-wire transmitters, see the full offering here.

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