Other Parts Discussed in Post: LM5017

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.

Output-isolated 4-wire sensor transmitter with local power supplyFigure 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.

4-wire sensor transmitter output stage design

Figure 2: 4-wire sensor transmitter output stage design

The positive output, I­OUT+, is connected to an +18V supply through a current-limiting circuit. The negative output terminal, IOUT-, 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 RSET resistor based on the input voltage, VIN, 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.

Complete output stage with digital isolator and DAC

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.

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

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.

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

 

Anonymous
  • Hello,

    I'm wondering about figures 1 & 2. Isn't there an issue regarding the ADC input voltage levels? The voltage accross the resistor must fit into the ADCs differential input voltage range, OK. But what about the maximum allowed voltage levels on the ADC inputs relative to the ADC ground (typically earth ground in typical non-isolated SMPS based DC systems)? E.g. what about the ADC input common mode voltage rating (referenced to ADC ground)? The resistor voltage (the IN+ and IN- voltage) seems to be floating relative to the ADC ground.

    Thanks,