Circuit Note CN-0270 Devices Connected/Referenced Circuits from the Lab reference designs are engineered and AD5700, Low Power HART Modem tested for quick and easy system integration to help solve todays AD5700-1 analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0270. 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 The circuit shown in Figure 1 uses the AD5700, the industrys 1 CN-0270 Evaluation Board (EVAL-CN0270-EB1Z) includes lowest power and smallest footprint HART -compliant IC modem USB-SWD/UART-EMUZ and the AD5420, a 16-bit current-output DAC, to form a complete Design and Integration Files HART-compatible 4 mA to 20 mA solution, typical of line-powered Schematics, Layout Files, Bill of Materials transmitter applications For additional space savings, the AD5700-1 offers a 0.5% precision internal oscillator. 10F *NC 10F 2.7V TO 5.5V 10.8V TO 36V *C2 C1 4.7nF 0.1F 0.1F 10k DV CAP2 CAP1 AV CC DD FAULT REFIN CLEAR DIGITAL REFOUT 0.1F INTERFACE LATCH SCLK AD5420 18 SDIN I OUT SDO 4mA TO 20mA UART INTERFACE CURRENT LOOP R L R SET GND 0.1F 15k C R H H V CC 8.2nF 27k TXD HART OUT C RXD L 4.7nF RTS REF CD 1F 1. 2M AD5700 300pF 150k ADC IP 150pF AGND DGND 1. 2M 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com 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) Fax: 781.461.3113 20122014 Analog Devices, Inc. All rights reserved. 10564-001CN-0270 Circuit Note This circuit adheres to the HART physical layer specifications as Determining the Values of the External Components defined by the HART Communication Foundation, for example, C1 and C2 can be used in conjunction with the digital slew rate the analog rate of change and noise during silence specifications. control functionality of the part to control the slew rate of the IOUT signal of the AD5420. In determining the absolute values of For many years, 4 mA to 20 mA communication has been used the capacitors, ensure that the FSK output from the modem is in process control instrumentation. This communication method is passed undistorted. Thus, the bandwidth presented to the modem reliable and robust, and offers high immunity to environmental output signal must pass the 1200 Hz and 2200 Hz frequencies. interference over long communication distances. A limitation, Figure 3 shows a circuit that achieves this requirement. In this however, is that only 1-way communication of one process variable at a time is possible. case, C2 is left open-circuit. C1 The development of the highway addressable remote transducer C2 (HART) standard provided highly capable 2-way digital CAP1 CAP2 AV DD communication, simultaneously with the 4 mA to 20 mA analog signaling used by traditional instrumentation equipment. This 40 allows for features such as remote calibration, fault interrogation, BOOST 4k and transmission of additional process variables. Put simply, HART is a digital two-way communication in which a 1 mA DAC 12. 5k peak-to-peak frequency-shift-keyed (FSK) signal is modulated I OUT on top of the 4 mA to 20 mA analog current signal. R L CIRCUIT DESCRIPTION Figure 1 shows the manner in which the AD5420 can be combined with the AD5700 HART modem and a UART interface to R C H R H C SET L V HART construct a HART-capable 4 mA to 20 mA current output. Such a circuit is popular in line-powered field transmitters in which a Figure 3. AD5420/AD5410 and AD5700 HART Modem Connection variety of input signals (RTD, TC, and ohm) are converted into The low-pass and high-pass filter circuitry is formed through a scalable analog, 4 mA to 20 mA output signal.The HART OUT the interaction of R , C , C , and C1, along with some internal H L H signal from the AD5700 is attenuated and ac-coupled into the circuitry in the AD5420. In calculating the values of these R pin of the AD5420. If the external R resistor is not being SET SET components, the low-pass and high-pass frequency cutoff point used, an alternative method of connecting the AD5420 and the targets were >10 kHz and <500 Hz, respectively. Figure 4 shows AD5700 can be found in Application Note AN-1065, where the a plot of the simulated frequency response, while Table 1 shows AD5700 HART modem output is coupled into the AD5420 via the effect on the frequency response of increasing each component the CAP2 pin. While the method described in this circuit note while the remaining component values are kept constant. requires the use of the external RSET resistor, it provides better 50 power supply rejection performance than the alternative 55 application note solution. The use of either solution results in 60 the AD5700 HART modem output modulating the 4 mA to 20 mA 65 analog current (as shown in Figure 2) without affecting the dc level 70 of the current. The diode protection circuitry (D1 to D4) is discussed 75 in more detail in the Transient Voltage Protection section. 80 85 901 = MARK0 = SPACE 1.2kHz 2.2kHz 95 100 105 110 1 10 100 1k 10k 100k START FREQUENCY (Hz) TXD Figure 4. Simulated Frequency Response STOP Table 1. Effect on Frequency Response of Individual HART OUT Component Value Increase Component C1 CH CL RH 8-BIT DATA + PARITY f (Hz) L Figure 2. AD5700/AD5700-1 Sample Modulator Waveform fH (kHz) No change No change No change G (dB) Rev. B Page 2 of 7 10564-002 I (I ) /HART (V) (dB) OUT 10564-004 10564-003