In any process control system, sensor transmitters collate data from pressure and temperature to flow and level, and relay this information to the programmable logic controller (PLC) or distributed control system.

These transmitters depend on the 4mA to 20mA signal to transmit data to the controller. Despite the emergence of standards such as IO-Link and Profibus, 4mA to 20mA offers resiliency over long distances, reliability, immunity to noise, and universal compatibility with every PLC system.

In this article, I will provide an overview of the 4mA to 20mA transmitter structure, its operating principles, and design alternatives for implementing this transmitter type using catalog semiconductor products.

4mA to 20mA transmitters are classified by power and number of wires: four, three and two wire. In this article, I will focus on the two-wire type.

The two-wire field transmitter in Figure 1 forms a current loop by connecting to a field supply and analog input module. The first subsystem in the field transmitter is the sense subsystem, which connects to the physical sensor, conditions its output, and converts the signal to a digital code for processing, including linearization and calibration. The second subsystem is the transmit subsystem, which powers the transmitter by extracting power from the loop, sends process data by converting the digital signal back to an analog signal, and controls the loop current. The transmitter transmits the signal by regulating current within the loop, acting as a voltage-controlled current source.

In Figure 2, an N-channel P-channel N-channel (NPN) transistor sources and regulates the current, whose base is controlled through an amplifier driven by a digital-to-analog converter (DAC). A wide input voltage low-dropout (LDO) regulator powers the different components by stepping down the loop voltage to the transmitter supply level. You can use a voltage reference if the DAC does not have an integrated reference, while Highway Addressable Remote Transducer (HART)-enabled transmitters require a HART modem.

The principle of operation is pretty simple: hold both inputs of the operational amplifier at virtual local ground. Whatever voltage R1 holds, Rsense also holds. With proper scaling, Rsense carries a scaled version of the R1 current. Given that Rsense current is nearly the whole current of the field transmitter (even for the sense part, not depicted in Figure 2), the DAC output controls the whole transmitter current. The NPN transistor and amplifier loop bypass the necessary current to complement any current used by the transmitter itself in order to achieve the required output current.

4mA to 20mA transmitter design considerations include:

• Low-power operation.
• A small footprint.
• Accuracy and low noise over the entire industrial temperature range.
• HART protocol support.
• Low cost.