The circuit shown in Figure 1 is a complete smart industrial loop-powered field instrument that provides a 4 mA to 20 mA analog output and a Highway Addressable Remote Transducer (HART ^® ) interface. HART is a digital bidirectional communication that modulates a 1 mA peak-to-peak frequency shift keying (FSK) signal on top of a 4 mA to 20 mA analog current signal. It enables numerous functions such as remote calibration, troubleshooting and process variable transmission; these functions are necessary in applications such as temperature and pressure control.

This circuit has been tested and verified for compatibility and is registered with the HART Communications Foundation (HCF). This successful registration allows circuit designers to use one or all components in a circuit with utmost confidence.

The circuit uses the ADuCM360 ultra-low power precision analog microcontroller, the AD5421 4mA to 20 mA 16-bit loop-powered digital-to-analog converter (DAC) , and the AD5700 HART-compatible IC modem, which is the industry's lowest power and smallest form factor .

Analog front-end interface

The ADuCM360 analog front end integrates a dual-channel, high-performance, 24-bit Σ-Δ analog-to-digital converter (ADC). Also integrated are programmable gain instrumentation amplifiers, precision bandgap voltage references, programmable current sources, flexible multiplexers, and many other features. The device allows direct interfacing with multiple analog sensors such as pressure sensor bridges, resistive temperature sensors, thermocouples, and other types of sensors for industrial applications.

Figure 1 shows an example circuit connecting the primary bridge sensor to an auxiliary resistive temperature sensor. The ADuCM360 has a flexible front end that allows various other configurations to meet the application requirements of various types of precision analog sensors.

Main sensor input

The on-chip ADC0 of the ADuCM360 measures the main sensor of the field instrument, represented as a bridge sensor in Figure 1. The sensor is connected to the analog input pins AIN0 and AIN1 through an RC filter network to enhance the system's immunity to electromagnetic interference. The common-mode filter bandwidth is approximately 16 kHz and the differential-mode bandwidth is 800 Hz.

The ADuCM360's VREF+ and VREF− reference voltage inputs detect the bridge's excitation voltage and initiate the circuit's ratiometric mode of operation, making the measurement independent of the exact value of the sensor supply voltage. If the application requires it, an on-chip ground switch can dynamically disconnect the bridge's excitation voltage, reducing power consumption.

Auxiliary sensor input

This circuit uses a 100Ω platinum (Pt) resistance temperature detector (RTD) as the auxiliary sensor. The RTD is able to detect the temperature of the main sensor and therefore, if necessary, temperature compensate the main sensor.

The ADuCM360 programmable current source provides current to the RTD through the AIN4 pin. ADC1 on the ADuCM360 measures the voltage of the RTD using the AIN3 and AIN2 pins configured as differential inputs. The exact current flowing through the RTD is detected using a precision resistor (RREF) and measured using the AIN7 pin of ADC1. ADC1 uses an on-chip bandgap voltage reference.

Digital data processing, algorithms and communications

All field instrument digital functions are provided by the ADuCM360 32-bit ARM Cortex™ M3 RISC processor, which integrates 128 kbytes of non-volatile fiash/EE memory, 8 kbytes of SRAM, and a wired (2× SPI , UART, I²C) communication peripherals.

The demonstration software performs initialization and configuration, processes data from analog inputs, controls analog outputs, and performs HART communications.

Analog output

The AD5421 integrates a 16-bit low-power precision DAC with a 4 mA to 20 mA loop-powered output driver to provide all the functionality required for analog output from field instruments.

AD5421 interfaces with the ADuCM360 controller through the SPI interface.

The AD5421 also integrates a range of diagnostic functions related to the 4 mA to 20 mA loop. The auxiliary ADC measures the voltage on the loop side of the instrument through a 20 MΩ/1 MΩ resistor divider connected to the VLOOP pin. The ADC also measures chip temperature via an internal sensor. The ADuCM360 controller can configure and read all diagnostic data of the AD5421, but the AD5421 can also operate autonomously.

For example, if the communication between the controller and the AD5421 fails, the AD5421 will automatically set its analog output to an alarm current of 3.2 mA after a period of time. This alarm current reports the failure of the on-site instrument to the host computer.

Any changes in the output current value are controlled by software to prevent interference with HART communication. (See the "Simulating Rate of Change" section).

HART communication

AD5700  integrates a complete HART FSK modem. The modem interfaces with the ADuCM360 controller via a standard UART interface with request-to-send (RTS) and carrier detect (CD) signals.

The HART output is trimmed to the desired amplitude through a 0.068 μF/0.22 μF capacitive voltage divider and coupled to the CIN pin of the AD5421, which is then used with the DAC output to drive and modulate the output current.

The HART signal coupled to the LOOP+ terminal is input to the ADC_IP pin of the AD5700 through a simple active RC filter. As the first stage, the RC filter serves as a band-pass filter for the HART demodulator and also enhances the system's ability to resist electromagnetic interference - which is very important for applications that work stably in harsh industrial environments.

The AD5700 low-power oscillator uses an external 3.8664 MHz crystal oscillator connected directly to the XTAL1 and XTAL2 pins to generate the HART modem clock.

Output protection

Transient voltage suppressors (TVS) protect the 4 mA to 20 mA HART interface from overvoltage. Its voltage rating should not exceed the AD5421 's absolute maximum voltage of 60 V at the REG _IN pin. Please note that TVS leakage current may affect the current output accuracy; therefore, when selecting this device, you need to pay attention to the leakage current under a certain loop voltage and temperature range.

An external depletion mode FET can be used in conjunction with the AD5421 to increase the maximum loop voltage.

This circuit is protected against polarity reversal by a pair of diodes in series with the loop output.

The ferrite bead is connected in series with the loop, and this series connection, along with the 4700 pF capacitor, improves the EMC performance of the system. Due to HART network specification limitations, do not use higher value capacitors at the loop endpoints.

The 4.7 V low-leakage Zener diode protects the AD5421's on-chip 50 Ω loop sense resistor from unexpected external voltages between the AD5421's COM pin and LOOP− pin (for example, when programming the ADuCM360 or debugging the circuit).

Power and power management

The complete field instrument circuit, including sensor drive current, must operate with limited power provided by the 4 mA to 20 mA loop. This is a common dilemma for all loop-powered field instrument designs. The circuit in Figure 1 provides an example of a low-power, high-performance solution. All three integrated circuits used in the application are designed for low power consumption, and the circuits rely on their respective integrated features to provide flexible power management structures and optimal performance loop power solutions.

The AD5421 operates from a 4 mA to 20 mA loop voltage, providing a regulated low voltage to the rest of the circuit. Depending on the needs of the circuit, the AD5421 REG _OUT voltage can be programmed from 1.8 V to 12 V. The circuit in Figure 1 uses the 3.3 V supply voltage option as an example of the input sensor used. However, because the ADuCM360 and AD5700 have a wider supply voltage range, different supply voltages can be used to meet application requirements.

REG _OUT RC filter (10 μF/10 Ω/10 μF) helps protect the sensor analog front end from any interference from the loop. It also prevents any interference generated by the circuitry (especially interference generated by controllers and digital circuits) from back-coupling into the loop, which is very important for reliable HART communication.

The AD5700 HART modem is powered through an additional RC filter (470 Ω/1 μF). This filter is important in loop-powered applications because it prevents the AD5700's current noise from coupling into the 4 mA to 20 mA loop output, which would otherwise affect HART communications. During silent testing, 4 mA to 20 mA loop noise performance was specifically addressed with HART in-band noise. The AD5700 modem uses an external crystal, choosing the lowest value in the achievable power dissipation range by connecting the 8.2 pF capacitors on XTAL1 and XTAL2 to ground.

The ADuCM360 has extremely flexible internal power management capabilities that provide many powering and clocking options for all internal modules and, when invoked by software, allow the optimal balance between required functionality, performance, and power consumption for a specific instrument application. Please refer to the ADuCM360 product page and AN-1111 application guide .

The analog front-end AVDD is powered through another filter (10 μF/ferrite bead/1.6 Ω/10 μF) to minimize power supply noise for low-voltage sensor signals for better performance.

The GND_SW ground switch pin of the ADuCM360 controls the excitation and power supply to the main sensor. When the instrument is powered on, the switch defaults to off. This default setting allows the system to be fully configured before turning on the sensor, including the appropriate power mode, thereby minimizing any power-up spikes that may be present on the 4 mA to 20 mA loop output.

Similarly, the auxiliary sensor is powered by the ADuCM360's programmable current source, so its power input is fully controlled by software.

ADuCM360 software

Basic code examples demonstrating the functionality and performance of this circuit can be found in the CN-0267 Design Support Package .

Code examples include basic HART slave command responses to demonstrate the functionality and features of the hardware. The code examples do not include the protocol layer for HART communication.