Solving The Signal-Chain Power Puzzle in 4-20mA Field Instruments

by Albert O'Grady and Michael Jackson 

The reliability and simplicity of 4-20mA and HART-enabled field instruments have made them a mainstay in industrial environments for decades, meaning they are unlikely to disappear from process control systems anytime soon. However, in the meantime, designers need to be aware of the challenges of powering the signal chains in these devices, given the limited amount of current available. This blog considers the relative merits of linear and switching regulators in this application and presents a ‘ready-to-go’ design for a low-power HART-enabled transmitter.

Linear Regulators

In traditional 4-20mA or HART-enabled field instruments, the current loop must also be able to provide power for signal chain components. Therefore, power management is not trivial since only 3.5 mA of current (the “low alarm” setting is at 0.5 mA below the 4 mA signal floor) is available to provide features, functionality, and processing.

Figure 1 shows the signal chain for a typical field-powered instrument powered by a linear regulator. The regulator output supplies power to the sensor and all other signal chain components. For devices with a HART interface, the noise contributed by a power supply is an essential consideration because the protocol specifies stringent performance specifications, making linear regulators the preferred choice from this perspective. However, using this approach, the amount of power available for components in the signal chain is limited to 3V * 3.5 mA = 10.5 mW.

Figure 1 Powering a field instrument signal chain using a linear regulator

Low Power HART Transmitter

Figure 2 shows the block diagram of the AD5700D2Z evaluation board, a loop-powered transmitter providing HART communication for two shared data channels measuring pressure (“0”) and temperature (“1”). This board, tested, verified, and registered as an approved solution by the HART Communication Foundation, features an AD5700 HART modem interfaced to an AD5421 16-bit, serial-input, loop-powered, 4-mA-to-20-mA DAC and an ADuCM360 microcontroller.

Figure 2 Loop-powered HART-enabled pressure and temperature transmitter

The low power consumption is an essential feature of this design. With a typical transmit current of only 124 µA (140µA max) and receive current of 86µA (115 µA max), this device does not place significant demands on the limited current budget.

Use a DC-DC Converter to Maximize Available Power 

On the other hand, for field instruments where it is essential to maximize available power (but where noise is less of a concern), using a DC-DC switching regulator, as shown in Figure 3, is preferable. Here, the power available at the input of the buck converter shown is also 10V * 3.5mA = 35mW. The ADP2360 is an 86% efficient buck regulator specially designed for use in field instruments and can provide almost three times the amount of output power compared to a linear regulator (35mW x 86% = 30mW). As a result, more than 10mA of current is available to power the sensor and other signal chain components in instruments operating from a typical 3V rail.

AD5700

Figure 3 Powering a field instrument signal chain using a DC-DC switching converter.

This approach makes extra current available for other purposes, like providing more processing power using a microcontroller from the MAX326xx series, for example, or including additional functional safety, security, diagnostics, or other features.

While the amount of loop current available to power the signal chain components in 4-20mA and HART-enabled field instruments is small, the blog has shown that designers still have the option to choose between using either a linear regulator or DC-DC switching converter, depending on their requirements.

Please visit here for more information about Analog Devices’ other field instrument signal chains. 

Find the next blog in this series here.