DEMO-AD5700D2Z

Circuit Note CN-0267 Devices Connected/Referenced Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0267. ADuCM360 Low Power, Precision Analog Microcontroller AD5421 16-Bit, Loop Powered, 4 mA to 20 mA DAC AD5700 Low Power HART Modem Complete 4 mA to 20 mA Loop Powered Field Instrument with HART Interface features such as remote calibration, fault interrogation, and transmission of process variables, which are necessary in applications such as temperature and pressure control. EVALUATION AND DESIGN SUPPORT Circuit Evaluation Board CN0267 Circuit Evaluation Board (DEMO-AD5700D2Z) Design and Integration Files Schematics, Layout Files, Bill of Materials, Code Example This circuit has been compliance tested, verified, and registered by the HART Communication Foundation (HCF). This successful registration provides circuit designers with a high level of confidence using one or all of the components in the circuit. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a complete smart industrial, loop powered field instrument with 4 mA to 20 mA analog output and a highway addressable remote transducer (HART®) interface. HART is a digital 2-way communication in which a 1 mA peakto-peak frequency-shift-keyed (FSK) signal is modulated on top of the standard 4 mA to 20 mA analog current signal. This allows The circuit uses the ADuCM360, an ultralow power, precision analog microcontroller, the AD5421, a 16-bit, 4 mA to 20 mA, loop powered digital-to-analog converter (DAC), and the AD5700, the industry’s lowest power and smallest footprint HARTcompliant IC modem. AD5421 L* 1.6Ω 10µF PRIMARY SENSOR 10µF 10µF 20MΩ REFOUT1 REFOUT2 VREF VREF– GND_SW AVDD 1kΩ AIN4 IEXC 0.1µF 1kΩ PT100 1kΩ 0.01µF AIN3 0.1µF AIN2 1kΩ ADC1 AGND SOUT SIN P0.5 P0.4 EXPOSED PAD AGND DGND AIN7 0.1µF SPI INTERFACE 0.47µF VREF SYNC SCLK WATCHDOG TIMER SDIN SDO LDAC COM 50Ω LOOP– CIN DVDD L* LOOP– 4.7V LOW LEAKAGE 470Ω 0.47µF AD5700 1µF VCC 3.8664MHz 0.01µF RREF 5.62kΩ 10ppm CS0 SCLK0 CORTEX MOSI0 M3 MISO0 + SRAM FLASH AVDD_ + REG DMA UART SPI DVDD_ I2C REG CLOCK RESET WATCHDOG 4700pF TVS 40V LOW LEAKAGE DAC ADC0 AIN1 1MΩ TEMPERATURE SENSOR 10µF 0.1µF 0.01µF SECONDARY SENSOR 1kΩ ADC 10µF IOVDD 0.01µF AIN0 0.1µF LOOP+ UART INTERFACE 0.068µF HART_OUT 0.22µF XTAL1 REF XTAL2 TXD RXD CD ADC_IP RTS REG_CAP DGND AGND 1µF 1µF 1.2MΩ 300pF 1.2MΩ 150kΩ 150pF 10551-001 1kΩ L* REGIN VLOOP DVDD VREF+ 1kΩ VOLTAGE REGULATOR 10µF DVDD ADuCM360 AVDD REGOUT 10Ω NOTES 1. L* = FERRITE BEAD, 0.3Ω @ DC, 1kΩ @ 100MHz. 2. THE ADuCM360 EXPOSED PAD IS CONNECTED TO DGND. Figure 1. 4 mA to 20 mA, Loop Powered Field Instrument with HART Interface (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. B Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2012–2013 Analog Devices, Inc. All rights reserved. CN-0267 Circuit Note CIRCUIT DESCRIPTION Digital Data Processing, Algorithm, and Communications Analog Front-End Interface 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 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 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 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 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). 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. 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. HART Communication 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 2 of 8 Circuit Note CN-0267 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 frontend. Aside from the second ADC, the ADuCM361 contains all the features of the ADuCM360. Rev. B | Page 3 of 8 CN-0267 Circuit Note The AD5421 can be connected via the protection directly to the loop. Alternatively, a depletion mode N-channel MOSFET can be connected between the AD5421 and the loop power supply, as shown in Figure 2. The use of the additional MOSEFT in this configuration keeps the voltage drop across the AD5421 at approximately 12 V, lowers the power dissipated in the AD5421 package, and therefore improves the 4 mA to 20 mA analog output accuracy. It also increases the maximum voltage allowed in the loop to the level of the MOSFET rating. The additional MOSFET has no effect on the HART communication. L LOOP+ 4700pF DRIVE 20MΩ VLOOP TVS 40V LOW LEAKAGE 1MΩ COM L LOOP– TO HART INPUT FILTER 10551-002 LOOP– 4.7V LOW LEAKAGE The circuit shown in Figure 1 is built on the DEMO-AD5700D2Z printed circuit board shown in Figure 3. The DEMO-AD5700D2Z circuit board includes some additional features for easy system evaluation. The 0.1 inch-pitch connector footprints allow optional primary and secondary sensor connections. There are test points for HART RTS and CD that may be needed for HART compliance tests. DN2540 BSP129 200kΩ Circuit Hardware Figure 3. DEMO-AD5700D2Z Printed Circuit Board (Pressure Sensor Not Included) AD5421 REGIN CIRCUIT EVALUATION AND TEST 10551-003 The ADuCM361 on-chip DAC with an external transistor can be used to control the 4 mA to 20 mA loop, refer to CN-0300 for details. Figure 2. MOSEFT Connected to the AD5421 Loop Power Supply The AD5700 is used with a 3.8664 MHz crystal in this circuit, which is the configuration achieving the lowest power consumption. Alternatively, the AD5700-1, with an integrated 0.5 %precision internal oscillator, can be used. The internal oscillator increases the modem power supply current by 225 µA maximum, compared to the crystal oscillator, but because no external crystal is needed, this option provides both cost savings and reduced board area requirements. For the applications that are not loop powered, the AD5410, AD5420, AD5422, or AD5755 are good choices for the 4 mA to 20 mA DAC. A connector on the edge of the DEMO-AD5700D2Z makes the ADuCM360 single wire and UART download/debug signals accessible allowing easy software development, code download, and in-circuit debugging and emulation. The connector, with a small header extender included with the DEMO-AD5700D2Z board, is compatible with all Analog Devices, Inc., Cortex-M3 based development tools, such as the EVAL-ADuCM360QSPZ evaluation kit (the evaluation kit is not included with the DEMO-AD5700D2Z board). These features are not shown in the simplified diagram in Figure 1; however, they can be seen in the complete circuit schematic in the CN-0267 Design Support Package. The design support package also includes a full field instrument C-code example, which enables complete verification and evaluation of all hardware blocks and features of the circuit, and a limited verification of the HART interface functionality. For detailed information about HART interface specifications and resources, contact the Hart Communication Foundation. HART Compliance The DEMO-AD5700D2Z has been verified to be compliant with HART FSK Physical Layer Specification (HCF_SPEC-054, Revision 8.1), using methods and equipment specified in the HART Physical Layer Test Specification (HCF_TEST-2, Revision 2.2). The board was submitted to the Hart Communication Foundation and was successfully registered. The registered circuit can be found on the HART Communication Foundation (HFC) web site in the product catalog as DEMOAD5700D2Z. The results of two of the tests involved the output noise during silence and the analog rate of change. Rev. B | Page 4 of 8 Circuit Note CN-0267 When a HART device is not transmitting (silence), do not couple noise onto the network. Excessive noise may interfere with reception of HART signals by the device itself or other devices on the network. The voltage noise measured across a 500 Ω load in the loop must contain no more than 2.2 mV rms of combined broadband and correlated noise in the HART extended frequency band. In addition, the noise should not exceed 138 mV rms outside the HART extended frequency band. This noise was measured by a true rms meter across the 500 Ω load. This noise was measured directly for the out-of-band noise and measured through the HCF_TOOL-31 filter for the in-band noise. An oscilloscope was also used to examine the noise waveform. The noise was measured at the worse condition, which was 4 mA output current. The captured noise waveform is shown in Figure 4, and the results are summarized in Table 1. The hardware slew-rate limit is set by the capacitance connected to the AD5421 CIN pin. When a large step change is required in the analog output current value, the ADuCM360 software splits the output current change sent to the AD5421 DAC into a number of smaller subsequent steps. This test was performed using an oscilloscope coupled to the 500 Ω load through the HCF_TOOL-31 filter. The result is shown in Figure 5. Waveform CH1 shows the periodic steps between 4 mA and 20 mA, sensed directly across the 500 Ω load. Waveform CH2 is the signal captured on the HCF_TOOL-31 filter output, amplified 10×, within the 150 mV peak limits. CH1 FREQ 8.123Hz? 1 CH2 OUTPUT OF FILTER × 10 MEASURE CH1 p-p 44.8mV CH2 p-p 254mV 2 CH2 MAX 134mV LIMIT = ±150mV CH1 CYC RMS 4.64mV? CH2 MIN –120mV CH1 NONE 1 MEASURE CH1 p-p 8.60V CH1 4mA TO 20mA ACROSS 500Ω CH1 5.00V CH2 50mV BW M 25.0ms CH1 6.80V 10551-005 Output Noise During Silence Test Figure 5. Analog Rate of Change Waveform Circuit Power Consumption CH1 NONE Two methods were used to evaluate the circuit power consumption performance. M 100ms CH1 –8.00mV