【Design Tips】Design Trade-offs for Loop-Powered Transmitters

Author: Derrick Hartmann (Application Engineer, Analog Devices)

Loop-powered transmitters have evolved from pure analog signal conditioners to highly flexible smart transmitters, but the design approach chosen still depends on the performance, functionality, and cost requirements of the system. This article provides three different transmitter reference designs.

In a loop-powered design, the 4 mA to 20 mA loop needs to provide both power and data, and the operating current of the system loop must be less than 4 mA.

In fact, a current of less than or equal to 3.6 mA is a typical target value, mainly used for low alarm currents in the loop. Other key factors in the design also need to consider target performance, function, size and cost. The first circuit we discuss (Figure 1) uses a purely analog signal chain.

Figure 1. Analog 4 mA to 20 mA loop-powered transmitter (reference CN0289).

The circuit measures a resistive bridge pressure sensor powered by a 5 V reference. The sensor signal is amplified by an instrumentation amplifier. Its voltage output is converted to a current through R1 and combined with the bias current generated through R2. This current flows through R3 and is amplified by an op amp configuration before forming a 4 mA to 20 mA output through R4. Since the current consumed by the entire transmitter is returned through R4, it is included in the 4 mA to 20 mA regulated current to power the circuit loop.

Using 0.1% resistors, the circuit can achieve a maximum accuracy of better than 1% at 25°C. Calibration can greatly improve the accuracy, and offset and gain calibration can be achieved by adjusting R2 and R1, respectively. However, the accuracy is still limited by the sensor performance and component temperature drift, because the circuit cannot be easily calibrated over temperature or sensor linearization.

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

All in all, this purely analog transmitter offers a simple, low-cost solution. However, the sensor cannot be linearized, it does not offer temperature calibration, and it does not provide diagnostic capabilities. Any changes to the sensor or output range will also require hardware changes.

Many of the shortcomings of purely analog circuits can be addressed by adding digital processing capabilities (as shown in Figure 2).

Figure 2. 4 mA to 20 mA loop-powered transmitter (referenced CN0145).

This circuit measures an RTD temperature sensor, powered by a current source, and performs a ratiometric measurement between the RTD and precision resistor R1. The RTD signal is conditioned using a PGA and converted to a digital output using a 24-bit Σ-Δ ADC. Calibration and linearization of the temperature sensor and 4 mA to 20 mA output are accomplished using data processing at the ARM 7 microcontroller.

This 4 mA to 20 mA output is controlled by a PWM signal to achieve 12-bit resolution. Although similar to the previous architecture, the output uses the non-inverting terminal of the op amp as the voltage control for the 4 mA to 20 mA loop. The 1.2 V reference voltage works with R2 to produce an equivalent current of 24 mA in the loop. This means that a control voltage of 0 V on the PWM produces a 24 mA output. The output current decreases as the control voltage on the PWM increases. For a current output of 4 mA, the PWM should be set to 500 mV. The advantage of this technique is that the PWM does not require buffering, which reduces power consumption and cost.

The power consumption of the entire RTD temperature transmitter was measured at 2.73 mA and 3.13 mA at 25°C and 85°C, respectively (excluding sensor excitation). This circuit meets the power consumption requirement, but if sensor excitation current or other diagnostics or additional features are included, there is almost no current available.

虽然成本略高于纯模拟变送器，但其完全实现了对传感器和输出的校准和线性化，使精度有了显著的提高。 它还可以更加灵活地实现诊断功能，并且在软件中考虑传感器类型变化也很容易。

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Design Trade-offs for Loop-Powered Transmitters

Figure 3. 4 mA to 20 mA loop-powered smart transmitter (referenced CN0267).

不过，仍存在一些局限性：4 mA到20 mA环路只能传输主变量(本例中为温度)，不能传输其他信息。 附加的诊断和系统功能虽在功耗预算范围内，却可能无法实现；更高的输入性能可能使4 mA到20 mA输出驱动器成为显著的系统误差来源。 能够克服这些限制的电路如图3所示。

This circuit is a true smart transmitter. In addition to providing excellent performance, it allows bidirectional communication on a 4 mA to 20 mA loop through the Highway Addressable Remote Transducer (HART®) protocol. The HART protocol can operate on traditional low-frequency loops by modulating a higher frequency 1.2 kHz, 2.2 kHz frequency-shift keyed (FSK) digital signal on the standard 4 mA to 20 mA analog signal. In addition, HART communication supports remote configuration transmission of diagnostic information, device parameters, and other measurement information.

As shown in Figure 3, the ADuCM360 independently measures the pressure sensor and RTD via a dual-channel, precision 24-bit Σ-∆ ADC with on-chip PGA. The low-power Cortex®-M3 core calibrates and linearizes the pressure sensor input, while the RTD is used for temperature compensation. The microcontroller also runs the HART protocol stack and uses the AD5700HART physical layer modem to communicate via UART. Finally, the microcontroller communicates with the AD5421 loop-powered DAC via SPI to control the 4 mA to 20 mA loop. The AD5421 is a fully integrated loop-powered 4 mA to 20 mA DAC; it includes a loop driver, a 16-bit DAC, a loop regulator, and diagnostic features.

Figure 4. HART communication.

With the ADC running at 50 SPS, the pressure sensor input achieves 18.5 bits of effective resolution. At the output, the AD5421 guarantees 16 bits of resolution and a maximum INL of 2.3 LSB.

The total circuit power consumption is typically 2.24 mA (excluding sensor excitation), of which the AD5421 consumes 225 µA, the AD5700 consumes 157 µA, and the ADuCM360 consumes 1.72 mA. The remainder is consumed by other circuits such as the on-chip LEDs. The 24-bit Σ-∆ ADC and PGA of the ADuCM360 are turned on, and the peripherals enabled include: on-chip reference, clock generator, watchdog timer, SPI, UART, timers, flash, SRAM, and a core operating at 2 MHz. HART communication consumes very little power, so other system diagnostics can be easily added to this system. Isolation is not addressed in the above circuits. In thermocouple transmitter applications, the exposed sensor may be directly bonded to a metal surface, so isolation is particularly important. Optocouplers are a solution, but they usually require a relatively large bias current to ensure reliable characteristics. The new ADuM124x and ADuM144x 2-channel/4-channel micropower isolators can meet these challenges.

Drawing only 0.3 µA per channel and 148 µA per Mbps of dynamic current, these devices enable isolation in systems that were previously not possible due to power constraints.

In summary, loop-powered transmitter designs can vary greatly based on performance, functionality, and cost. The three solutions described above offer different design trade-offs, from the simplest analog transmitter to a feature-rich smart transmitter. In smart transmitter designs, new low-power products are bringing performance, functionality, and integration to previously unattainable levels.

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