Circuit Note CN-0179 Devices Connected/Referenced 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/CN0179. AD8657 18 V, Precision, Micropower CMOS Rail-toRail I/O Dual Operational Amplifier ADR02 Ultracompact Precision 5 V Voltage Reference AD5641 2.7 V to 5.5 V, <100 µA, 14-Bit nanoDAC, SPI Interface 4-20 mA Low Power, 14-Bit, Process Control Current Loop Transmitter EVALUATION AND DESIGN SUPPORT budget for higher power devices, such as microcontrollers and digital isolators. The circuit output is 0 mA to 20 mA of current, and it operates on a single supply from 8 V to 18 V. The 4 mA to 20 mA range is usually mapped to represent the input control range from the DAC or micro-controller, while the output current range of 0 mA to 4 mA is often used to diagnose fault conditions. Circuit Evaluation Boards CN0179 Circuit Evaluation Board (EVAL-CN0179-PMDZ) System Demonstration Platform (EVAL-SDP-CB1Z) SDP Interposer Board (SDP-PMD-1B1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS The 14-bit, 5 V AD5641 requires 75 µA typical supply current. The AD8657 is a rail-to-rail input/output dual op amp and is one of the lowest power amplifiers currently available in the industry (22 µA per amplifier over the full supply voltage and input common-mode range) with high operating voltage of up to 18 V. The ADR02 ultracompact precision 5 V voltage reference requires only 650 µA. Together, these three devices consume a typical supply current of 747 µA. The circuit in Figure 1 is a 4 mA-to-20 mA current loop transmitter for communication between a process control system and its actuator. Besides being cost effective, this circuit offers the industry’s low power solution. The 4 mA-to-20 mA current loop has been used extensively in programmable logic controllers (PLCs) and distributed control systems (DCS’s), with digital or analog inputs and outputs. Current loop interfaces are usually preferred because they offer the most cost effective approach to long distance noise immune data transmission. The combination of the low power AD8657 dual op amp, AD5641DAC, and ADR02 reference allows more power The circuit has a 12-pin Pmod™ digital interface (Digilent specification). 18V VSY 18V VOUT 10µF 0.1µF VREF 5V 0.1µF GND 10µF 1/2 18V ZENER BZX84C18 AD8657 A2 PMOD SCLK SDIN SI2319DS-T1-E3 100Ω BA S21LT1 IOUT VDD SYNC VDAC AD5641 GND GND R2 100Ω 0.1% 25ppm/°C VOUT 1/2 AD8657 2N700T A1 J1 RLOAD 250Ω RSENSE 2.49kΩ 0.1% 25ppm/°C GND 09371-001 ADR02 VIN R1 1kΩ 0.1% 25ppm/°C P1 Figure 1. Low Power 4 mA-to-20 mA Process Control Current Loop (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. A 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 ©2010–2014 Analog Devices, Inc. All rights reserved. CN-0179 Circuit Note (1) 20 18 where: VREF is the output of ADR02 and the power supply to the AD5641. D is the decimal equivalent of the binary code that is loaded to the AD5641. OUTPUT CURRENT (mA) 16 The DAC output voltage sets the current flowing through the sense resistor, RSENSE, where (2) 6 4 2 DAC INPUT CODE 16384 09371-002 15360 14336 13312 11264 12288 9216 10240 8192 7168 6144 5120 4096 3072 2048 0 Figure 2. 0 mA to 20 mA Output Current Figure 3 shows the output current error plot in percent full-scale range. The overall worst-case error is approximately 0.35% measured over the output range between Code 256 and Code 16,128. Rev. A | Page 2 of 5 0.25 0.20 0.15 0.10 0.05 0 16384 15360 14336 09371-003 DAC INPUT CODE Figure 3. Output Current Error Plot 13312 12288 11264 10240 9216 8192 –0.05 7168 The ADR02 is an ultracompact, precision 5 V voltage reference. With an 18 V input voltage, quiescent current is only 650 µA, typical. It has an initial accuracy of 0.06% (B-grade) and 10 μV p-p voltage noise. Connecting a 0.1 μF ceramic capacitor to the output is highly recommended to improve stability and filter out low level voltage noise. An additional 1 µF to 10 µF electrolytic, tantalum, or ceramic capacitor in parallel can improve load transient response. A 1 μF to 10 μF electrolytic, tantalum or ceramic capacitor can also be connected to the input to improve transient response in applications where the supply voltage may fluctuate. An additional 0.1 μF ceramic capacitor should be connected in parallel to reduce supply noise. 0.30 6144 In addition, this circuit solution requires a rail-to-rail input amplifier. The AD8657 dual op amp is an excellent choice, with low power and rail-to-rail features. The op amp operates with a typical supply current of 22 µA/amplifier over the specified supply voltage and input common-mode voltage. It also offers excellent noise and bandwidth per unit of current. The AD8657 is one of the lowest power amplifiers that operate on supplies of up to 18 V. 0.35 5120 The AD5641 is a 14-bit DAC from the nanoDAC family and operates from the 5 V output voltage of the ADR02 reference. It has an on-chip precision output buffer that is capable of swinging from rail-to-rail (within 10 mV), thus allowing a high dynamic output range. With a supply voltage of 5 V, AD5641 consumes a typical 75 µA of supply current. 4096 With VDAC ranging from 0 V to 5 V, the circuit generates a current output from 0 mA to 20 mA. 8 3072 (3) 10 2048 IOUT = IR2 = (VDAC/RSENSE ) × ( R1/R2) IDEAL CURRENT MEASURED CURRENT 12 1024 The current through RSENSE varies from 0 mA to 2 mA as a function of VDAC. This current develops a voltage across R1 and sets the voltage at the noninverting input of the AD8657 amplifier (A2). The A2 AD8657 closes the loop and brings the inverting input voltage to the same voltage as the noninverting input. Therefore, the current flowing through R1 is mirrored by a factor of 10 to R2. This is represented by Equation 3. 14 0 ISENSE = VDAC/RSENSE READING ERROR (%FSR) VDAC = VREF × (D/214) Figure 2 shows the linearity of the system, that is the measured output current from the circuit DAC input code from 0 to full-scale. 1024 For industrial and process control modules, 4 mA-to-20 mA current loop transmitters are used as a means of communication between the control unit and the actuator. Located at the control unit, the 14-bit AD5641 DAC produces an output voltage, VDAC, between 0 V and 5 V as a function of the input code. The code is set via an SPI interface. The ideal relationship between the input code and output voltage is given by Bypass capacitors (not shown in Figure 1) are required. In this case, a 10 µF tantalum capacitor in parallel with a 0.1 µF ceramic capacitor should be placed on each power pin of each dual op amp. Details of proper decoupling techniques can be found in Tutorial MT-101. 0 CIRCUIT DESCRIPTION Circuit Note CN-0179 Figure 4 shows the calibrated output current error plot. Removing the gain and offset error from Figure 3, the accuracy is better than 0.05% measured over the output range between Code 256 and Code 16,128. 0.05 READING ERROR (%FSR) 0.03 0.01 –0.01 CIRCUIT EVALUATION AND TEST This circuit uses the EVAL-CN0179-PMDZ circuit board, the EVAL-SDP-CB1Z system demonstration platform (SDP) evaluation board and the SDP-PMD-IB1Z, a Pmod interposer board for the EVAL-SDP-CB1Z. The SDP and the SDP-PMDIB1Z boards have 120-pin mating connectors, allowing the quick setup and evaluation of the circuit’s performance. In order to evaluate the EVAL-CN0179-PMDZ board using the SDP-PMD-IB1Z and the SDP, the EVAL-CN0179-PMDZ is connected to the SDP-PMD-IB1Z by a standard 100 milspaced, 25 mil square, right angle 12 pin-Pmod header connector. Information and details regarding how to use the evaluation software for data capturing and proper hardware installation can be found in the CN0179 Software User Guide. –0.03 Equipment Required 15360 16384 • 09371-004 DAC INPUT CODE 14336 13312 11264 12288 9216 10240 8192 7168 6144 5120 4096 3072 2048 0 1024 –0.05 Figure 4. Calibrated Output Current Error Plot The data in Figure 3 and Figure 4 shows larger errors at zero and full-scale because the output buffer of the AD5641 DAC limits when its output is within 10 mV of either supply rail. The region between Code 0 and Code 255 as well as the region between Code 16,129 and Code 16,384 are therefore excluded from the linearity specifications. This corresponds to approximately 0 V to 80 mV and 4.92 V to 5.00 V at the DAC voltage output; and 0 mA to 0.32 mA and 19.68 mA to 20.00 mA referenced to the current output. The test data was taken using the board shown in Figure 6. Complete documentation for the system can be found in the CN-0179 Design Support package. Information and details regarding how to use the evaluation software for data capturing and proper hardware installation can be found in the CN0179 Software User Guide. Test Setup and Measurements COMMON VARIATIONS For a 16-bit resolution solution, consider the AD5660 or AD5662, respectively. The 16 V CMOS ADA4665-2 op amp is another option to replace the AD8657. It lower cost and has lower voltage noise at the expense of a higher supply current. When selecting amplifiers for this application, always ensure that the input common-mode voltage range and the supply voltage are not exceeded. • • • • • • • • PC with a USB port and Windows® XP, Windows® Vista (32-bit), or Windows® 7 (32-bit) EVAL-CN0179-PMDZ circuit evaluation board EVAL-SDP-CB1Z SDP evaluation board SDP-PMD-IB1Z CN0179 evaluation software Agilent E36311A dual dc power supply or equivalent Agilent 3458A multimeter or equivalent +6 V wall wart A GPIB-to-USB cable adapter (only required for capturing analog data from the output and transferring it to the PC) The circuit was tested using the test setup in Figure 5. A photograph of the board is shown in Figure 6. A jumper should not be connected to the J1 terminals when driving an external current loop. The jumper connects the internal 250 Ω load and should be used when making voltage measurements. Rev. A | Page 3 of 5 CN-0179 Circuit Note GPIB INTERFACE CURRENT METER 6V POWER SUPPLY OR WALL WART USB USB CONA 120 PINS J1 J4 J3 PMOD EVAL-SDP-CB1Z SDP-PMD-IB1Z SDP BOARD INTERPOSER BOARD 18V POWER SUPPLY P1 PMOD IOUT EVAL-CN0179-PMDZ 09371-005 PC 09371-006 Figure 5. Functional Diagram of Test Setup Figure 6. Photo of EVAL-CN0179-PMDZ Board Rev. A | Page 4 of 5 Circuit Note CN-0179 LEARN MORE Data Sheets CN0179 Design Support Package: http://www.analog.com/CN0179-DesignSupport AD8657 Data Sheet AN-345 Application Note, Grounding for Low- and HighFrequency Circuits, Analog Devices. AD5641 Data Sheet ADR02 Data Sheet AD5662 Data Sheet AN-347 Application Note, Shielding and Guarding: How to Exclude Interference-Type Noise, Analog Devices. AD5660 Data Sheet Colm Slattery, Derrick Hartmann, and Li Ke, “PLC Evaluation Board Simplifies Design of Industrial Process Control Systems,” Analog Dialogue (April 2009). Jung, Walt. Op Amp Applications, Analog Devices. Also available as Op Amp Applications Handbook, Elsevier. Kester, Walt. 2005. The Data Conversion Handbook. Chapters 3 and 7. Analog Devices. MT-015 Tutorial, Basic DAC Architectures II: Binary DACs. Analog Devices. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of “AGND” and “DGND.” Analog Devices. MT-035, Op Amp Inputs, Outputs, Single-Supply, and Rail-toRail Issues, Analog Devices. ADA4665-2 Data Sheet REVISION HISTORY 3/14—Rev. 0 to Rev. A Changed AD5621 to AD5641....................................... Throughout Changed ADR125 to ADR02........................................ Throughout Changes to Figure 1 .......................................................................... 1 Changes to Circuit Description Section, Figure 2, and Figure 3 ............................................................................................... 2 Changes to Common Variations Section ....................................... 3 Added Figure 4 and Circuit Evaluation and Test Section ............ 3 Added Figure 5 and Figure 6 ........................................................... 4 Changes to Learn More Section and Data Sheets Section ........... 5 11/10—Revision 0: Initial Version MT-101 Tutorial, Decoupling Techniques. Analog Devices. (Continued from first page) Circuits from the Lab reference designs 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 reference designs in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab reference designs 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 reference designs at any time without notice but is under no obligation to do so. ©2010–2014 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN09371-0-3/14(A) Rev. A | Page 5 of 5