In the realm of industrial automation and control, 2-wire loop-powered 4 to 20mA field transmitters are very common for their reliable and accurate signal transmission over long distances. The current loop's ability to maintain signal integrity over several hundred meters, or even kilometers, is largely attributed to the current signal's robustness against the electrical noise that can be induced into long cable runs. The current-based signal, as opposed to voltage signal, is less susceptible to voltage drops and the interference from electromagnetic sources, making sure a stable and reliable communication pathway between sensors and control systems.

However, the very nature of industrial sites often requires sensors, such as thermocouples, to be in direct contact with grounded metallic structures, while the control systems they communicate with can reference a different ground potential. This disparity can result in ground loop currents, which, due to the extensive distances involved, can significantly degrade measurement accuracy by introducing unwanted noise and potential offsets into the signal. To mitigate these issues, electrical isolation becomes imperative.

Figure 2-11 shows a typical block diagram of an isolated two-wire loop powered 4 to 20mA temperature transmitter system, including the current flow used for the communication bus and power for the system, which is designed to convert sensor measurements into a 4 to 20mA loop current signal. Under normal operating conditions, the total current consumption of the transmitter’s transmitter must be less than the actual loop current. Power savings achieved from any individual sub-circuit, including the isolated DC/DC, can be redistributed to other circuit blocks of the system. It allows for optimization of other functionality, ultimately leading to a more efficient end product. Due to the limited supply current budget, it is crucial that a highly efficient power design is imperative. This application note will focus on the isolated DC/DC designs which encased in the red box in Figure 2-11, and the design 3 is more suitable for situations where there is already an MCU on the non-isolated side of the system, which the location is shown in the blue box. Additionally, the flexibility of these designs also allows them to be customized for various applications that demand small size and highly efficient power conversion and isolated data transmission, especially when they must operate from power sources with restricted current availability.