Designing output isolated 4-wire sensor transmitters

In my last post, I discussed the basic structure of 4-wire sensor transmitters and how they differ from 2-wire and 3-wire sensor transmitters. In this post, I will discuss the construction of a locally powered output-isolated 4-wire sensor transmitter like the one shown in Figure 1. Locally powered 4-wire sensor transmitters are popular in applications where the wires must run long distances and the sensor consumes >4mA, preventing the use of a 2-wire transmitter. A common example is electromagnetic flow metering.

Figure 1: Output-isolated 4-wire sensor transmitter with local power supply

The output stage design of a typical 4-wire transmitter is usually simpler than 2- or 3-wire transmitter output stages because the sense resistor in the 4-wire analog input module is floating. Therefore, you can use a simple current-sink topology like the one shown in Figure 2. You could also use a current-source topology, but that would require a two-stage design similar to those found in 3-wire transmitters.

Figure 2: 4-wire sensor transmitter output stage design

The positive output, I_­OUT+, is connected to an +18V supply through a current-limiting circuit. The negative output terminal, I_OUT-, is connected to the drain of an N-type metal-oxide semiconductor (NMOS) transistor. An operational amplifier (op amp) drives the gate of the NMOS transistor to control the current through the R_SET resistor based on the input voltage, V_IN, resulting in the V-I transfer function shown in Equation 1:

In an output-isolated transmitter, the output stage must be completely isolated from the sensor and power supply. This requires the generation of an isolated power supply from the local supply, as well as a way to send the sensor information across the isolation barrier. You can accomplish this by using a digital isolator and digital-to-analog converter (DAC), as shown in Figure 3.

Figure 3: Complete output stage with digital isolator and DAC

The final part of the output-isolated 4-wire sensor transmitter is the isolated power supply for the output stage. You can achieve an isolated power supply in many ways, depending on the input and output voltages. Figure 4 shows an example solution with a +24V input and +18 and +5V outputs. The LM5017 buck regulator creates an isolated +20V output from the +24V sensor-supply input. The TPS7A49 low-dropout regulator (LDO) creates the +18V output and the TPS7A1650 LDO creates the +5V output.  For more information about power supply designs using the LM5017, check out this blog written in TI's Power House.

Figure 4: Isolated power supply for an output-isolated 4-wire transmitter

Figure 5 shows a complete output-isolated 4-wire sensor transmitter with local power supply.

Figure 5: Output-isolated 4-wire sensor transmitter with local power supply

In this post, I described an example circuit design of an output-isolated 4-wire sensor transmitter constructed from an isolated power-supply solution, a digital isolator, a DAC and an op amp circuit. In my next posts, I’ll describe power-isolated and fully isolated 4-wire sensor transmitters.

Additional resources

• Check out these TI Designs reference designs for 2-wire transmitters:
  • Bridge Sensor Signal Conditioner with Current Loop Output, EMC Protection (TIPD126).
  • Isolated Loop Powered Thermocouple Transmitter Reference Design (TIDA-00189).
  • Low Cost Loop-Powered 4-20mA Transmitter EMC/EMI Tested Reference Design (TIPD158).
• Read these 3-wire blog posts from my colleague and precision DAC expert Kevin Duke:
  • An overview of analog outputs and architectures.
  • The evolution of 3-wire analog outputs.
• Find commonly used analog design formulas in the new Analog Engineer’s Pocket Reference e-book by my colleagues Art Kay and Tim Green