Three solutions for loop-powered transmitter design

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

Loop-powered transmitters have evolved from pure analog signal conditioners to highly flexible smart transmitters, but the design method chosen still depends on the performance, functionality, and cost requirements of the system.

In the loop power supply design, the 4mA to 20mA loop needs to provide power and data at the same time, and the operating current of the system loop must be less than 4mA. In fact, a current less than or equal to 3.6mA is a typical target value, which is mainly used for loops with low alarm currents. Other key factors in the design also need to consider target performance, function, size, and cost. The DY circuits we are discussing (Figure 1) use a purely analog signal chain.

Three solutions for loop-powered transmitter design
Figure 1: Analog 4mA to 20mA loop-powered transmitter (refer to CN0289).

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

With a resistance of 0.1%JD, the ZGJD of this circuit can be better than 1% at 25°C. Calibration can greatly improve JD, and offset and gain calibration can be achieved respectively by adjusting R2 and R1. However, JD is still limited by sensor performance and component temperature drift, because the circuit cannot easily calibrate temperature or sensor linearization. The power consumption of this circuit is less than 1.9mA (excluding sensor excitation), which is far below the target value of 4mA.

All in all, this purely analog transmitter provides a simple low-cost solution. However, the sensor cannot be linearized, it does not provide temperature calibration, nor does it provide diagnostic functions. Any change in sensor or output range also requires hardware changes.

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

Three solutions for loop-powered transmitter design
Figure 2: 4mA to 20mA loop-powered transmitter (refer to CN0145)

This circuit measures an RTD temperature sensor, uses a current source to supply power, and performs a ratio measurement between the RTD and the precision resistor R1. The RTD signal can be conditioned by PGA and converted to digital output by a 24-bit Σ-? ADC. Using ARM7 microcontroller for data processing, calibration and linearization of the temperature sensor and 4mA to 20mA output can be realized.

The 4mA to 20mA output is controlled by a PWM signal, which can achieve 12-bit resolution. Although similar to the previous architecture, the output uses the non-inverting terminal of the operational amplifier as the voltage control of the 4mA to 20mA loop. The 1.2V reference voltage source cooperates with R2 to generate an equivalent current of 24mA in the loop. This means that a PWM control voltage of 0V produces a 24mA output. The output current decreases as the control voltage on the PWM increases. For 4mA current output, PWM should be set to 500mV. The advantage of this technology is that PWM does not need to be buffered, which reduces power consumption and cost.

The measured value of the power consumption of the entire RTD temperature transmitter at 25°C and 85°C is 2.73mA and 3.13mA (excluding sensor excitation). This circuit meets the power consumption requirements, but if it includes sensor excitation current or other diagnostic or additional features, there is almost no current available.

Although the cost is slightly higher than that of a pure analog transmitter, it fully realizes the calibration and linearization of the sensor and output, which significantly improves JD. It can also implement diagnostic functions more flexibly, and it is easy to consider sensor type changes in the software.

However, there are still some limitations: the 4mA to 20mA loop can only transmit the main variable (temperature in this case), and cannot transmit other information. Additional diagnostics and system functions may not be possible even though they are within the power budget; higher input performance may make 4mA to 20mA output drivers a significant source of system error. The circuit that can overcome these limitations is shown in Figure 3.

Three solutions for loop-powered transmitter design
Figure 3: 4mA to 20mA loop-powered smart transmitter (refer to CN0267)

This circuit is a real smart transmitter. In addition to providing excellent performance, it also allows two-way communication on the 4mA to 20mA loop through the addressable remote sensor high-speed channel (HART?) protocol. By modulating a higher frequency 1.2kHz, 2.2kHz frequency shift keying (FSK) digital signal on a standard 4mA to 20mA analog signal, the HART protocol can run in a traditional low frequency loop. In addition, HART communication supports remote configuration transmission of diagnostic information, device parameters and other measurement information.

As shown in Figure 3, ADuCM360 performs independent measurements on the pressure sensor and RTD through a dual-channel, precision 24-bit Σ-? ADC with on-chip PGA. The low-power Cortex?-M3 core can calibrate and linearize the pressure sensor input, and the RTD is used for temperature compensation. The microcontroller also runs the HART protocol stack and uses the AD5700 HART physical layer modem to communicate via UART. ZH, the microcontroller communicates with the AD5421 loop-powered DAC via SPI to control the 4mA to 20mA loop. AD5421 is a fully integrated loop-powered 4mA to 20mA DAC; it includes a loop driver, 16-bit DAC, loop regulator, and diagnostic features.

Figure 4: HART communication

When the ADC is running at 50 SPS, the pressure sensor input can achieve an effective resolution of 18.5 bits. At the output end, the AD5421 is guaranteed to provide 16-bit resolution and an INL of ZD2.3 LSB.

The power consumption of the entire circuit is typically 2.24mA (excluding sensor excitation), among which the power consumption of AD5421 is 225μA, AD5700 is 157μA, ADuCM360 is 1.72mA, and the rest is the power consumption of other circuits such as on-chip LEDs. The 24-bit Σ-? ADC and PGA of ADuCM360 are in the on state, and the peripheral enable includes: on-chip reference voltage source, clock generator, watchdog timer, SPI, UART, timer, flash memory, SRAM and work A core with a frequency of 2MHz. The power consumption of HART communication is extremely low, so other system diagnostics and other functions can be easily added to the system.

None of the above circuits involve isolation issues. In thermocouple transmitter applications, the exposed sensor may be directly bound to the 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 devices ADuM124x and ADuM144x 2-channel/4-channel micro-power isolator can meet these challenges.

The quiescent current and dynamic current of each channel of these devices are only 0.3μA and 148μA/Mbps, respectively. They can achieve isolation in the system, which was previously not possible due to power consumption limitations.

In short, loop-powered transmitter design can vary a lot based on performance, function, and cost. The above three solutions provide different design trade-offs, from simple analog transmitters to feature-rich smart transmitters. In the design of smart transmitters, new low-power products have improved performance, functionality, and integration to levels that were previously unattainable.

The Links:   LM150X07 G150XTN036

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