To complement the 2-wire and 3-wire sensor transmitter blogs that Kevin Duke and I have already published on Precision Hub, my next few blog topics will be on 4-wire sensor transmitters. The blogs will explain 4-wire transmitter basics, the circuit structure of 4-wire output stages, and provide examples of each isolation scheme available with 4-wire transmitters.

The 4-wire sensor transmitter is probably the least well-known of the current-loop sensor transmitter circuit types. These transmitters fit market needs for applications that require additional transmitter isolation options that aren’t possible with 2- and 3-wire transmitters. Figure 1 shows a basic representation of a 4-wire sensor transmitter based on the American National Standards Institute (ANSI)/International Society of Automation (ISA)-50.1-1982 standard.

Unlike the 2- and 3-wire transmitter representations shown in Figure 2, the 4-wire circuit has separate paths for the power current and signal current. Also, the 4-wire receiver does not share a common return (GND) with the power supply. This allows for several new isolation schemes, including fully isolated, power-isolated and output-isolated transmitters that expand on the input-isolated and non-isolated topologies we described for 2-wire and 3-wire transmitters.

While non-isolated and input-isolated systems exist for 2- and 3-wire sensor transmitters, these isolation schemes are not possible when designing with 4-wire transmitters. This is because non-isolated and input-isolated transmitters do not require isolation between the power supply and the output transmitter and receiver.

I’d like to note that input isolated 3-wire transmitters can typically be connected to 4-wire analog input modules by connecting the negative input (Input-) to GND. Figure 3 shows an example of the transmitter and receiver wiring connections.

The first and most basic form of a 4-wire transmitter design that I’ll cover is an output-isolated 4-wire transmitter. In an output-isolated 4-wire transmitter, the sensor input and power supply share a common GND, while the output transmitter is powered from an isolated supply derived from the sensor supply. Figure 4 is a simple block diagram of an output-isolated 4-wire transmitter.

Output-isolated 4-wire transmitters can also be powered from a local power supply instead of being powered from the 4-wire analog input module (see Figure 5). The sensor and power supply still share a common GND while the transmitter is isolated from them.

This 4-wire transmitter scheme is a popular way to reduce the number of wires that must travel long distances back to the input module when there is easily accessible power near the sensor.

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

In my post next month, I’ll go through the circuit-level design of an output-isolated 4-wire sensor transmitter.

Additional resources

• Check out these TI Designs reference designs for 2-wire transmitters:
• Bridge sensor signal conditioner with current loop output, EMC protection (TIPD126).
• Low cost loop-powered 4-20mA transmitter EMC/EMI tested reference design (TIPD158).
• Isolated loop powered thermocouple transmitter reference design (TIDA-00189).
• Read these 3-wire blog posts from Kevin Duke:
• An overview of analog outputs and architectures.
• The evolution of 3-wire analog outputs.
• Find commonly used analog design formulas in a new pocket reference by Art Kay and Tim Green.