EVAL-CN0270-EB1Z

Circuit Note CN-0270 Circuits from the Lab® reference designs 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/CN0270. Devices Connected/Referenced AD5700, Low Power HART® Modem AD5700-1 AD5420 16-Bit 4 mA to 20 mA DAC Complete 4 mA to 20 mA HART Solution EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards CN-0270 Evaluation Board (EVAL-CN0270-EB1Z) includes USB-SWD/UART-EMUZ Design and Integration Files Schematics, Layout Files, Bill of Materials The circuit shown in Figure 1 uses the AD5700, the industry’s lowest power and smallest footprint HART1-compliant IC modem and the AD5420, a 16-bit current-output DAC, to form a complete HART-compatible 4 mA to 20 mA solution, typical of line-powered transmitter applications For additional space savings, the AD5700-1 offers a 0.5% precision internal oscillator. 10µF *NC 10µF 2.7V TO 5.5V 10kΩ DVCC CAP2 10.8V TO 36V C1 4.7nF *C2 0.1µF 0.1µF AVDD CAP1 FAULT REFIN CLEAR DIGITAL INTERFACE REFOUT 0.1µF LATCH SCLK SDIN AD5420 18Ω IOUT SDO UART INTERFACE 4mA TO 20mA CURRENT LOOP RL RSET GND 0.1µF 15kΩ VCC TXD HART_OUT RH 27kΩ CH 8.2nF CL 4.7nF RXD RTS REF 1µF AD5700 1.2MΩ 300pF ADC_IP AGND DGND 1.2MΩ 150kΩ 150pF 10564-001 CD Figure 1. AD5420 HART-Enabled Circuit Simplified Schematic 1 HART® is a registered trademark of the HART Communication Foundation. Rev. B Circuits from the Lab reference designs 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 toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) 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 ©2012–2014 Analog Devices, Inc. All rights reserved. CN-0270 Circuit Note This circuit adheres to the HART physical layer specifications as defined by the HART Communication Foundation, for example, the analog rate of change and noise during silence specifications. For many years, 4 mA to 20 mA communication has been used in process control instrumentation. This communication method is reliable and robust, and offers high immunity to environmental interference over long communication distances. A limitation, however, is that only 1-way communication of one process variable at a time is possible. Determining the Values of the External Components C1 and C2 can be used in conjunction with the digital slew rate control functionality of the part to control the slew rate of the IOUT signal of the AD5420. In determining the absolute values of the capacitors, ensure that the FSK output from the modem is passed undistorted. Thus, the bandwidth presented to the modem output signal must pass the 1200 Hz and 2200 Hz frequencies. Figure 3 shows a circuit that achieves this requirement. In this case, C2 is left open-circuit. The development of the highway addressable remote transducer (HART) standard provided highly capable 2-way digital communication, simultaneously with the 4 mA to 20 mA analog signaling used by traditional instrumentation equipment. This allows for features such as remote calibration, fault interrogation, and transmission of additional process variables. Put simply, HART is a digital two-way communication in which a 1 mA peak-to-peak frequency-shift-keyed (FSK) signal is modulated on top of the 4 mA to 20 mA analog current signal. C1 C2 CAP1 CAP2 AVDD 40Ω BOOST 4kΩ 12.5kΩ DAC IOUT "1" = MARK 1.2kHz RSET CH RH CL VHART Figure 3. AD5420/AD5410 and AD5700 HART Modem Connection The low-pass and high-pass filter circuitry is formed through the interaction of RH, CL, CH, and C1, along with some internal circuitry in the AD5420. In calculating the values of these components, the low-pass and high-pass frequency cutoff point targets were >10 kHz and