Loop-powered transmitters have progressed from purely analog signal conditioners to highly flexible smart transmitters, but the chosen design approach still depends on a system’s performance, functionality and cost requirements. This article presents three difference bench-tested transmitter designs.

In loop-powered designs, the 4-20mA loop provides both power and data, so the system must operate on less than 4mA loop current. In fact, 3.6mA or lower is a more typical target as this represents a low alarm current on the loop. Other key considerations for a design are the target performance, functionality, size, and cost.

The first circuit we’ll discuss (Figure 1) uses a purely analog signal chain.

Figure 1. Analog 4-20mA loop-powered transmitter (reference to Circuits from the Lab® Reference Design)

This circuit measures a resistive bridge pressure sensor, which is excited by a 5V reference. An instrumentation amplifier gains up the sensor signal. Its voltage output is converted to a current by R1, and is summed with an offset current generated through R2. This current flows through R3 and is amplified, via the op amp configuration, through R4 to form the 4-20mA output. As the current consumed by the entire transmitter returns through R4, it is included in the regulated 4-20mA current, making the circuit loop powered.

Using 0.1% resistors this circuit can achieve better than 1% max accuracy at 25°C. Calibration would greatly increase the accuracy; allowing adjustment of R2 and R1 would cater for offset and gain calibration respectively. However, the accuracy is still limited by sensor performance and component drift over temperature, as the circuit does not easily allow for calibration over temperature or sensor linearization.

This circuit consumes less than 1.9mA (excluding sensor excitation), which is well below the 4mA target.

In summary, this purely analog transmitter allows for a simple, low-cost solution, but the sensor cannot be linearized, it does not offer calibration over temperature, it provides no diagnostics, and any changes to the sensor or output range would require hardware changes.

A number of the drawbacks of the purely analog circuit can be resolved by adding digital processing capability as shown in Figure 2.

Figure 2 . 4-20mA loop-powered transmitter (reference to Circuits from the Lab® Reference Design)

This circuit measures an RTD temperature sensor, which is excited by a current source. A ratiometric measurement is taken between the RTD and precision resistor R1. The RTD signal is conditioned using a PGA, whose output is converted to digital by a 24-bit sigma-delta ADC. This data can be manipulated using the ARM 7 microcontroller, which can calibrate and linearize the temperature sensor and the 4-20mA output.

The 4-20mA output is controlled via a PWM signal, which is able to achieve 12bit resolution. Though similar to the previous architecture, the output uses an op amp’s noninverting terminal as the control voltage for the 4-20mA loop. A 1.2V reference, along with R2 generates a current equivalent to 24mA on the loop. This means a 0V control voltage from the PWM results in a 24mA output. As the control voltage on the PWM increases, the output current decreases. For a 4mA current output, the PWM should be programmed to 500mV. The advantage of this technique is that the PWM does not need to be buffered, reducing both current consumption and cost.

The current consumption of the entire RTD temperature transmitter was measured as 2.73mA at 25°C and 3.13mA at 85°C (excluding sensor excitation). This circuit meets the power consumption requirements, but there is little current left for any additional diagnostics or features once the sensor excitation current is added.

Though slightly higher cost than the purely analog transmitter, the ability to fully calibrate and linearize the sensor and output offers significantly improved accuracy. It also has greater flexibility to allow for diagnostics, and changes in sensor type can easily be accounted for in software.

There are still some limitations though. Some of these include; the 4-20mA loop can only transmit the primary variable, in this case temperature, and no other information; additional diagnostics and system functionality may not be possible while staying within the power budget; and with higher input performance the 4-20mA output driver may become a significant source of system error.

A circuit that overcomes these limitations is shown in figure 3.

Figure 3 . 4-20mA loop-powered smart transmitter (reference to Circuits from the Lab® Reference Design)

This circuit is truly a smart transmitter. As well as providing exceptional performance, it allows bidirectional communication over the 4-20mA loop via the Highway Addressable Remote Transducer (HART®) protocol. The HART protocol operates over a traditional, low-frequency loop by modulating a higher frequency 1.2 kHz, 2.2 kHz frequency shift keyed (FSK) digital signal over the standard 4-20mA analog signals. HART communication enables, among other things, remote configuration and transmission of diagnostic information, device parameters and additional measurement information.

Per Figure 3, a pressure sensor and RTD are measured independently on the ADuCM360 via dual precision 24-bit sigma-delta ADCs with on board PGA. The low-power cortex M3 core calibrates and linearizes the pressure sensor input, and the RTD is used for temperature compensation. The microcontroller also runs the stack for the HART protocol and communicates via UART with the AD5700 HART physical layer modem. Lastly, the microcontroller communicates with the AD5421 loop-powered DAC via SPI to control the 4-20mA loop. The AD5421 is a fully integrated loop-powered 4-20mA DAC; it includes the loop driver, 16-bit DAC, loop regulator, and diagnostic features.

With the ADC running at 50sps, the pressure sensor input was able to achieve 18.5bit effective resolution. On the output side, the AD5421 provides guaranteed 16-bit resolution and INL of 2.3LSBs maximum.

The whole circuit consumes 2.24mA typ (excluding sensor excitation), with 225µA consumed by the AD5421, 157µA by the AD5700, 1.72mA by the ADuCM360, and the remaining current by other circuitry such as an on-board LED. The ADuCM360 is running with both 24-bit sigma delta ADCs and PGAs active and the following peripherals enabled: on-chip reference, clock generator, watchdog timer, SPI, UART, timers, FLASH, SRAM, and the core running at 2MHz. This extremely low power consumption along with HART communication means that additional system diagnostics and functionality can easily be added to this system.

One aspect not discussed in any of the above circuits is isolation. Isolation is especially useful in thermocouple transmitter applications where the exposed sensor may be bonded directly to a metal surface. Optocouplers are one solution, though they typically require a relatively large bias current to ensure reliable behavior. New devices that overcome such challenges are the ADuM124x and ADuM144x 2/4 channel micropower isolators. These devices consume a mere 0.3µA quiescent current per channel and 148 µA/Mbps dynamic current per channel. They enable isolation in systems where previously it was not an option due to power constraints.

In conclusion, loop-power transmitter designs can vary significantly in performance, functionality, and cost. The three solutions discussed provide different design trade-offs, starting from the simplest analog transmitter to a feature rich smart transmitter. New low-power products enable a level of performance, functionality, and integration not previously achievable in smart transmitter designs.

Wow. I actually built the same thing, an optically isolated HART transmitter for a vortex flow meter, about 100 years ago. At least it seems that long to me. The circuits here are much easier than building it by hand. Does the new HART modem do the wave fo

Hi Doug, The HART modem used (AD5700) has a sinusoidal output (controlled via UART). In terms of isolation, likely the most straightforward is to isolate the UART interface. It is also possible to isolate the analog signals using a transformer (albeit a pr

“sir i need this type of unified analogue signals of constant voltage of 4-20ma with galvanic separation of circuits”

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