DEMO-AD5700D2Z

CN-0267_V01 AD | Alldatasheet

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information and/or support, visit www.analog.com/CN0267. Fax: 781.461.3113 ©2012–2013 Analog Devices, Inc. All rights reserved. and a highway addressable remote transducer (HART®) interface. applications such as temperature and pressure control. confidence using one or all of the components in the circuit. Figure 1. 4 mA to 20 mA, Loop Powered Field Instrument with HART Interface (Simplified Schematic: All Connections and Decoupling Not Shown)

1. THE ADuCM360 EXPOSED PAD IS CONNECTED TO DGND.

Rev. B | Page 2 of 8 CIRCUIT DESCRIPTION Analog Front-End Interface The ADuCM360 analog front-end incorporates dual, high performance 24-bit sigma-delta (Σ-Δ) analog-to-digital converters (ADCs). It also integrates programmable gain instrumentation amplifiers, a precision band-gap reference, programmable current sources, a flexible multiplexer, and many other features. It allows a direct interface to multiple analog sensors, such as pressure sensor bridges, resistive temperature sensors, thermocouples, and many other types of sensors used in the industry. The circuit in Figure 1 shows an example connection for a primary bridge type sensor and a secondary resistive temperature sensor; however the ADuCM360 flexible front-end allows many other configurations to accommodate any type of precision analog sensor application. Primary Sensor Input The ADuCM360 on-chip ADC0 measures the field instrument primary sensor, shown as a bridge transducer in Figure 1. The sensor connects to the analog input pins, AIN0 and AIN1, via an RC filter network for improved system electromagnetic immunity. The common-mode filter bandwidth is approximately 16 kHz, and the differential-mode bandwidth is 800 Hz. The ADuCM360 VREF+ and VREF− voltage reference inputs sense the bridge excitation voltage and enable the circuit to work in a ratiometric mode, making the measurement independent of the exact value of the sensor power supply voltage. The on-chip ground switch can dynamically disconnect the bridge excitation and save power when required by the application. Secondary Sensor Input The circuit uses a platinum (Pt) 100 Ω resistive temperature device (RTD) as a secondary sensor. The RTD can sense the temperature of the primary sensor and thus allow for temperature compensation of the primary sensor if required. The ADuCM360 programmable current source supplies the RTD via the AIN4 pin. The ADC1 on the ADuCM360 measures the voltage across the RTD using the AIN3 and AIN2 pins configured as a differential input. The exact value of the current flowing through the RTD is sensed by a precision resistor (RREF) and is measured by the ADC1 using the AIN7 pin. The ADC1 uses the on-chip, band-gap voltage reference. Digital Data Processing, Algorithm, and Communications All the field instrument digital functions are provided by the ADuCM360 32-bit ARM Cortex™ M3 RISC processor, with integrated 128 k bytes of nonvolatile flash/EE memory, 8 k bytes of SRAM, and an 11-channel direct memory access (DMA) controller that supports wired (2× SPI, UART, I²C) communication peripherals. The demonstration software performs the initialization and configuration, processes data from the analog inputs, controls the analog output, and performs the HART communication. Analog Output The AD5421 integrates a low power precision 16-bit DAC with a 4 mA to 20 mA, loop powered output driver and provides all functions required for the field instrument analog output. The AD5421 interfaces with the ADuCM360 controller via the SPI interface. The AD5421 also includes a range of diagnostic functions related to the 4 mA to 20 mA loop. The auxiliary ADC can measure the voltage across the instruments loop terminals via the 20 MΩ/1 MΩ resistive divider connected to the VLOOP pin. The ADC can also measure the chip temperature via the integrated sensor. The ADuCM360 controller can configure and read all the diagnostics of the AD5421, but the AD5421 can also operate autonomously. As an example, if the communication between the controller and the AD5421 fails, the AD5421 automatically sets its analog output to a 3.2 mA alarm current after a defined period. This alarm current indicates to the host that the field instrument failed to operate. The software controls any change of the output current from one value to another to prevent disturbance of the HART communication. (See the Analog Rate of Change section). HART Communication The AD5700 integrates a complete HART FSK modem. The modem is connected to the ADuCM360 controller via a standard UART interface, complemented by request to send (RTS) and carrier detect (CD) signals. The HART output is scaled to the required amplitude by the 0.068 µF/0.22 µF capacitive divider and coupled to the AD5421 CIN pin, where it is combined with the DAC output to drive and modulate the output current. The HART input is coupled from LOOP+ via a simple passive RC filter to the AD5700 ADC_IP pin. The RC filter works as the first stage, band-pass filter for the HART demodulator and also improves the system electromagnetic immunity, which is important for robust applications working in harsh industrial environments. The AD5700 low power oscillator generates the clock for the HART modem with a 3.8664 MHz external crystal connected directly to the XTAL1 and XTAL2 pins.

Rev. B | Page 3 of 8 Output Protection A transient voltage suppressor (TVS) protects the 4 mA to 20 mA HART interface from overvoltage. Its voltage rating should prevent exceeding the AD5421 absolute maximum voltage of 60 V on the REGIN pin. Note that the TVS leakage current can affect the current output accuracy; therefore, pay attention to the leakage current at a given loop voltage and temperature range when selecting this component. An external depletion-mode FET can be used with the AD5421 to increase the loop voltage maximum The circuit is protected against reversed polarity by a pair of diodes in series with loop output. The ferrite beads in series with the loop together with the 4700 pF capacitor improve the system EMC performance. Do not use a higher capacitance across the loop terminals because of the HART network specifications. The 4.7 V, low leakage, Zener diode protects the AD5421 on-chip, 50 Ω loop sense resistor in the event of an accidental external voltage between the AD5421 COM pin and LOOP− pin (for example, when programming the ADuCM360 or debugging the circuit). Power Supplies and Power Management The complete field instrument circuitry, including the sensor drive current, must operate on the limited amount of power available from the 4 mA to 20 mA loop. This is a common challenge in any loop powered field instrument design. The circuit in Figure 1 provides an example of delivering both a low power and high performance solution. All three integrated circuits used in the application are designed for low power, and the circuit leverages their integrated features to deliver a flexible power management structure and an optimum loop-powered solution. The AD5421 is powered by the 4 mA to 20 mA loop voltage and provides a regulated low voltage for the rest of the circuit. The AD5421 REGOUT voltage is pin programmable from 1.8 V to 12 V depending on circuit requirements. The circuit in Figure 1 uses the 3.3 V supply voltage option as an example for the input sensors used. However, the ADuCM360 and the AD5700 have a wider power supply voltage range; therefore, a different power supply voltage can be used to suit the application. The REGOUT RC filter (10 µF/10 Ω/10 µF) helps to prevent any interference coming from the loop affecting the sensor analog front-end. It also prevents any interference generated by the circuit, specifically by the controller and the digital circuitry, from coupling back to the loop, which is important for a reliable HART communication. The AD5700 HART modem is supplied through an additional RC filter (470 Ω/1 µF). This filter is very important in the loop powered application because it prevents current noise from the AD5700 from coupling to the 4 mA to 20 mA loop output, which would otherwise affect the HART communication. The 4 mA to 20 mA loop noise performance is specifically addressed by the HART in-band, noise during silence test. The AD5700 modem uses the external crystal with 8.2 pF capacitors to ground on the XTAL1 and XTAL2 pins, which is the option using the least possible power. The ADuCM360 has very flexible internal power management, with many options for powering and clocking all the internal blocks and, when utilized by the software, allows an optimal balance between the required function, performance, and power for the specific instrument application. Refer to the ADuCM360 product page and the AN-1111 Application Note. The analog front-end AVDD is supplied from another filter (10 µF/ferrite bead/1.6 Ω/10 µF) to minimize power supply noise for better performance with respect to low voltage sensor signals. The GND_SW ground switch pin of the ADuCM360 controls the excitation/power supply for the primary sensor. The switch is off as a default at the instrument power up. This default allows the system to be fully configured, including appropriate power modes, before turning on the sensor, and thus minimizes any possible power-up spikes on the 4 mA to 20 mA loop output. Similarly, the secondary sensor is supplied from the programmable current source of the ADuCM360, and therefore, its power is fully controlled by the software. ADuCM360 Software A basic code example that demonstrates the functionality and performance of the circuit can be found in the CN-0267 Design Support Package. The code example includes a basic HART slave command response to demonstrate the hardware function and capability. However, the code example does not include the protocol layers of the HART communication. COMMON VARIATIONS The ADuCM360 has a high performance and very flexible analog front-end, with 12 analog input pins and extra pins for voltage reference and ground switch. It allows direct interface to multiple analog sensors of varying types, such as any resistive bridge sensors, resistive temperature sensors, or thermocouples. Therefore, do not limit the field instrument solution to temperature-compensated pressure measurement only because it can be used for almost any sensor field instrument. The ADuCM361 can be used as an alternative to the ADuCM360 in applications that need only one Σ-Δ ADC in the analog front- end. Aside from the second ADC, the ADuCM361 contains all the features of the ADuCM360.

• REGOUT = 3.3 V
• ADuCM360 M3 core clock = 2 MHz
• Both ADCs converting at 50 samples per second
• ADC0 has both buffers on and gain = 8
• ADC1 has both buffers on and gain = 16
• RTD excitation current = 200 µA
• SPI communicating to AD5421 with serial clock = 100 kHz
• HART communicating The circuit with all relevant analog and digital blocks, including the input sensor, consumes power supply current within the budget allowed at the minimum 4 mA loop current.

Table 2. Power Supply Current from AD5421, REGOUT = 3.3V determined by the specifications of the ADuCM360. the noise and related resolution performance of the system.

• The first test was performed with a standard pressure sensor (Honeywell 24PCDFA6D) soldered directly on the board.
• A second test was performed with the primary input signal generated by a set of fixed and variable resistors, as shown in Figure 7.

Figure 7. Primary Input Signal Generated by a Set of Resistors

Rev. B | Page 8 of 8 LEARN MORE CN-0267 Design Support Package: http://www.analog.com/CN0267-DesignSupport CN-0270, Complete 4 mA to 20 mA HART Solution CN-0278, Complete 4 mA to 20 mA HART Solution with Additional Voltage Output Capability CN-0300, Complete Closed-Loop Precision Analog Microcontroller Thermocouple Measurement System with 4 mA to 20 mA Output AN-1111, Options for Minimizing Power Consumption When Using the ADuCM360/ADuCM361 HART® Communication Foundation Data Sheets and Evaluation Boards ADuCM360 Data Sheet and Evaluation Board AD5421 Data Sheet and Evaluation Boards AD5700 Data Sheet and Evaluation Board

REVISION HISTORY

11/13—Rev. A to Rev. B 2/13—Rev. 0 to Rev. A 12/12—Revision 0: Initial Version (Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab circuits in the design of your product, no other license is granted by imp lication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so. ©2012–2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN10551-0-11/13(B)