Three Transmitter Reference Designs for Loop-Powered Transmitters
“Loop-powered transmitters have evolved from purely 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, a 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.
“
Loop-powered transmitters have evolved from purely 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, a 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 loops with low alarm currents. Other key factors in the design also need to consider target performance, function, size and cost.
The yi-th circuit we discuss (Figure 1) employs a purely analog signal chain.
Figure 1. Analog 4 mA to 20 mA loop powered transmitter (reference CN0289).
The circuit measurement is 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 current through R1, and the bias current generated through R2 is combined. This current flows through R3, is amplified by an op-amp configuration, and then passes through R4 to form a 4 mA to 20 mA output. Since the current drawn by the entire transmitter is returned through R4, it is included in the regulated 4 mA to 20 mA current to power the circuit loop.
With a 0.1% jerk Resistor, the circuit’s very high jerk at 25°C can be better than 1%. Calibration can greatly improve the accuracy, and offset and gain calibration can be achieved by adjusting R2 and R1, respectively. However, the degree of precision is still limited by sensor performance and component temperature drift because the circuit cannot easily calibrate for temperature or sensor linearization.
The circuit consumes less than 1.9 mA (excluding sensor excitation), well below the target value of 4 mA.
All in all, this pure analog transmitter provides a simple low-cost solution. However, the sensor cannot be linearized, it does not provide temperature calibration, and it does not provide diagnostics. Any change in sensor or output range also requires changing hardware. Many of the shortcomings of purely analog circuits can be addressed by adding digital processing power (as shown in Figure 2).
Figure 2. 4 mA to 20 mA loop powered transmitter (reference CN0145).
This circuit measures an RTD temperature sensor, powered by a current source, with a ratiometric measurement between the RTD and precision resistor R1. The RTD signal can be conditioned with a PGA and converted to a digital output by a 24-bit sigma-delta ADC. Calibration and linearization of the temperature sensor and 4 mA to 20 mA output is accomplished using data processing at the ARM 7 microcontroller.
The 4 mA to 20 mA output is controlled by a PWM signal, enabling 12-bit resolution. Although similar to the previous architecture, the output uses the noninverting terminal of the op amp as the voltage control for the 4 mA to 20 mA loop. The 1.2 V reference voltage source in conjunction with R2 produces an equivalent current of 24 m in the loop. This means that a PWM control voltage of 0 V 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 PWM does not need buffering, which reduces power consumption and cost.
The power consumption of the entire RTD temperature transmitter is measured at 2.73 mA and 3.13 mA at 25°C and 85°C, respectively (excluding sensor excitation). The circuit meets the power dissipation requirements, but if sensor excitation current or other diagnostics or additional features are included, little current is available.
Although the cost is slightly higher than the pure analog transmitter, it fully realizes the calibration and linearization of the sensor and output, which has significantly improved the accuracy. It also allows for more flexibility in implementing diagnostic functions, and it is easy to account for sensor type changes in the software.
Figure 3. 4 mA to 20 mA loop powered smart transmitter (reference CN0267).
However, there are some limitations: the 4 mA to 20 mA loop can only transmit the primary variable (temperature in this case) and no other information. Additional diagnostics and system functions may not be available while within the power budget; higher input performance may make the 4 mA to 20 mA output driver a significant source of system error. A circuit that can overcome these limitations is shown in Figure 3.
This circuit is a true smart transmitter. In addition to providing superior performance, it also allows bidirectional communication over a 4 mA to 20 mA loop via the Highway Addressable Remote Transducer (HART) protocol. By modulating a higher frequency 1.2 kHz, 2.2 kHz frequency shift keying (FSK) digital signal on a standard 4 mA to 20 mA analog signal, the HART protocol operates on a traditional low frequency loop. In addition, HART communication enables remote configuration transfer of diagnostic information, device parameters and other measurement information.
As shown in Figure 3, the ADuCM360 makes independent measurements of the pressure sensor and RTD via a dual-channel, precision 24-bit sigma-delta ADC with an on-chip 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 HART protocol stack and communicates over UART using the AD5700 HART physical layer modem. Later, 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, 16-bit DAC, loop regulator, and diagnostic features.
Figure 4. HART communication.
When the ADC operates at 50 SPS, the pressure sensor input achieves an effective resolution of 18.5 bits. On the output side, the AD5421 is guaranteed to provide 16-bit resolution and a large 2.3 LSB INL. The power consumption of the entire circuit is typically 2.24 mA (excluding sensor excitation), where the AD5421
The power consumption is 225 μA, AD5700 is 157 μA, ADuCM360 is 1.72 mA, and the rest is the power consumption of other circuits such as on-chip LEDs. The 24-bit Σ-Δ ADC and PGA of the ADuCM360 are on, and peripheral enables include: on-chip reference, clock generator, watchdog timer, SPI, UART, timer, flash, SRAM, and working cores at 2 MHz. The extremely low power consumption of HART communication makes it easy to add features such as additional system diagnostics to the system.
The isolation problem is not involved in any of the above circuits. In thermocouple transmitter applications, where the exposed sensor may be directly bonded to the metal surface, isolation is especially important. Optocouplers are one solution, however they usually require a relatively large bias current to ensure reliable performance. The new devices, the ADuM124x and ADuM144x 2-channel/4-channel micropower isolators, address these challenges.
The quiescent and dynamic currents per channel of these devices are only 0.3 µA and 148 µA/Mbps, respectively. They enable isolation in the system that was previously impossible due to power consumption constraints.
In summary, loop-powered transmitter designs can vary widely in terms of performance, functionality, and cost. The three solutions described above offer different design trade-offs, from very simple analog transmitters to feature-rich smart transmitters. In smart transmitter design, new low-power products bring performance, functionality, and integration to previously unattainable levels.
About the Author
Derrick Hartmann is a systems applications engineer in the Industrial and Instrumentation Group at Analog Devices. His area of focus is process control applications with a background in industrial DACs. He is a graduate of the University of Limerick, Ireland, with a bachelor’s degree in electrical engineering. Contact information:.
Loop-powered transmitters have evolved from purely 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, a 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 loops with low alarm currents. Other key factors in the design also need to consider target performance, function, size and cost.
The yi-th circuit we discuss (Figure 1) employs a purely analog signal chain.
Figure 1. Analog 4 mA to 20 mA loop powered transmitter (reference CN0289).
The circuit measurement is 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 current through R1, and the bias current generated through R2 is combined. This current flows through R3, is amplified by an op-amp configuration, and then passes through R4 to form a 4 mA to 20 mA output. Since the current drawn by the entire transmitter is returned through R4, it is included in the regulated 4 mA to 20 mA current to power the circuit loop.
With a 0.1% jerk resistor, the circuit’s very high jerk at 25°C can be better than 1%. Calibration can greatly improve the accuracy, and offset and gain calibration can be achieved by adjusting R2 and R1, respectively. However, the degree of precision is still limited by sensor performance and component temperature drift because the circuit cannot easily calibrate for temperature or sensor linearization.
The circuit consumes less than 1.9 mA (excluding sensor excitation), well below the target value of 4 mA.
All in all, this pure analog transmitter provides a simple low-cost solution. However, the sensor cannot be linearized, it does not provide temperature calibration, and it does not provide diagnostics. Any change in sensor or output range also requires changing hardware. Many of the shortcomings of purely analog circuits can be addressed by adding digital processing power (as shown in Figure 2).
Figure 2. 4 mA to 20 mA loop powered transmitter (reference CN0145).
This circuit measures an RTD temperature sensor, powered by a current source, with a ratiometric measurement between the RTD and precision resistor R1. The RTD signal can be conditioned with a PGA and converted to a digital output by a 24-bit sigma-delta ADC. Calibration and linearization of the temperature sensor and 4 mA to 20 mA output is accomplished using data processing at the ARM 7 microcontroller.
The 4 mA to 20 mA output is controlled by a PWM signal, enabling 12-bit resolution. Although similar to the previous architecture, the output uses the noninverting terminal of the op amp as the voltage control for the 4 mA to 20 mA loop. The 1.2 V reference voltage source in conjunction with R2 produces an equivalent current of 24 m in the loop. This means that a PWM control voltage of 0 V 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 PWM does not need buffering, which reduces power consumption and cost.
The power consumption of the entire RTD temperature transmitter is measured at 2.73 mA and 3.13 mA at 25°C and 85°C, respectively (excluding sensor excitation). The circuit meets the power dissipation requirements, but if sensor excitation current or other diagnostics or additional features are included, little current is available.
Although the cost is slightly higher than the pure analog transmitter, it fully realizes the calibration and linearization of the sensor and output, which has significantly improved the accuracy. It also allows for more flexibility in implementing diagnostic functions, and it is easy to account for sensor type changes in the software.
Figure 3. 4 mA to 20 mA loop powered smart transmitter (reference CN0267).
However, there are some limitations: the 4 mA to 20 mA loop can only transmit the primary variable (temperature in this case) and no other information. Additional diagnostics and system functions may not be available while within the power budget; higher input performance may make the 4 mA to 20 mA output driver a significant source of system error. A circuit that can overcome these limitations is shown in Figure 3.
This circuit is a true smart transmitter. In addition to providing superior performance, it also allows bidirectional communication over a 4 mA to 20 mA loop via the Highway Addressable Remote Transducer (HART) protocol. By modulating a higher frequency 1.2 kHz, 2.2 kHz frequency shift keying (FSK) digital signal on a standard 4 mA to 20 mA analog signal, the HART protocol operates on a traditional low frequency loop. In addition, HART communication enables remote configuration transfer of diagnostic information, device parameters and other measurement information.
As shown in Figure 3, the ADuCM360 makes independent measurements of the pressure sensor and RTD via a dual-channel, precision 24-bit sigma-delta ADC with an on-chip 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 HART protocol stack and communicates over UART using the AD5700 HART physical layer modem. Later, 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, 16-bit DAC, loop regulator, and diagnostic features.
Figure 4. HART communication.
When the ADC operates at 50 SPS, the pressure sensor input achieves an effective resolution of 18.5 bits. On the output side, the AD5421 is guaranteed to provide 16-bit resolution and a large 2.3 LSB INL. The power consumption of the entire circuit is typically 2.24 mA (excluding sensor excitation), where the AD5421
The power consumption is 225 μA, AD5700 is 157 μA, ADuCM360 is 1.72 mA, and the rest is the power consumption of other circuits such as on-chip LEDs. The 24-bit Σ-Δ ADC and PGA of the ADuCM360 are on, and peripheral enables include: on-chip reference, clock generator, watchdog timer, SPI, UART, timer, flash, SRAM, and working cores at 2 MHz. The extremely low power consumption of HART communication makes it easy to add features such as additional system diagnostics to the system.
The isolation problem is not involved in any of the above circuits. In thermocouple transmitter applications, where the exposed sensor may be directly bonded to the metal surface, isolation is especially important. Optocouplers are one solution, however they usually require a relatively large bias current to ensure reliable performance. The new devices, the ADuM124x and ADuM144x 2-channel/4-channel micropower isolators, address these challenges.
The quiescent and dynamic currents per channel of these devices are only 0.3 µA and 148 µA/Mbps, respectively. They enable isolation in the system that was previously impossible due to power consumption constraints.
In summary, loop-powered transmitter designs can vary widely in terms of performance, functionality, and cost. The three solutions described above offer different design trade-offs, from very simple analog transmitters to feature-rich smart transmitters. In smart transmitter design, new low-power products bring performance, functionality, and integration to previously unattainable levels.
About the Author
Derrick Hartmann is a systems applications engineer in the Industrial and Instrumentation Group at Analog Devices. His area of focus is process control applications with a background in industrial DACs. He is a graduate of the University of Limerick, Ireland, with a bachelor’s degree in electrical engineering. Contact information:.
View more : IGBT modules | LCD displays | Electronic Components