2 4mA to 20mA transmitter basics

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.