Circuit Note CN-0179 Devices Connected/Referenced 18 V, Precision, Micropower CMOS Rail-to- AD8657 Circuits from the Lab reference designs are engineered and Rail I/O Dual Operational Amplifier tested for quick and easy system integration to help solve todays Ultracompact Precision 5 V Voltage analog, mixed-signal, and RF design challenges. For more ADR02 Reference information and/or support, visit www.analog.com/CN0179. 2.7 V to 5.5 V, <100 A, 14-Bit nanoDAC, SPI AD5641 Interface 4-20 mA Low Power, 14-Bit, Process Control Current Loop Transmitter budget for higher power devices, such as microcontrollers and EVALUATION AND DESIGN SUPPORT digital isolators. The circuit output is 0 mA to 20 mA of current, Circuit Evaluation Boards and it operates on a single supply from 8 V to 18 V. The 4 mA to CN0179 Circuit Evaluation Board (EVAL-CN0179-PMDZ) System Demonstration Platform (EVAL-SDP-CB1Z) 20 mA range is usually mapped to represent the input control SDP Interposer Board (SDP-PMD-1B1Z) range from the DAC or micro-controller, while the output Design and Integration Files current range of 0 mA to 4 mA is often used to diagnose fault Schematics, Layout Files, Bill of Materials conditions. CIRCUIT FUNCTION AND BENEFITS The 14-bit, 5 V AD5641 requires 75 A typical supply current. The circuit in Figure 1 is a 4 mA-to-20 mA current loop The AD8657 is a rail-to-rail input/output dual op amp and is transmitter for communication between a process control one of the lowest power amplifiers currently available in the system and its actuator. Besides being cost effective, this circuit industry (22 A per amplifier over the full supply voltage and offers the industrys low power solution. The 4 mA-to-20 mA input common-mode range) with high operating voltage of up current loop has been used extensively in programmable logic to 18 V. The ADR02 ultracompact precision 5 V voltage reference controllers (PLCs) and distributed control systems (DCSs), requires only 650 A. Together, these three devices consume a with digital or analog inputs and outputs. Current loop typical supply current of 747 A. interfaces are usually preferred because they offer the most cost The circuit has a 12-pin Pmod digital interface (Digilent effective approach to long distance noise immune data specification). transmission. The combination of the low power AD8657 dual op amp, AD5641DAC, and ADR02 reference allows more power 18V V P1 SY 18V GND R1 R2 1k 100 ADR02 V 0.1% 0.1% REF 5V 25ppm/C 25ppm/C V V IN OUT 10F 0.1F 0.1F 10F GND 18V ZENER BZX84C18 1/2 AD8657 SI2319DS-T1-E3 100 A2 BA S21LT1 VDD SYNC IOUT V DAC V AD5641 PMOD SCLK OUT 1/2 J1 2N700T AD8657 SDIN GND A1 R LOAD 250 GND R SENSE 2. 49k 0.1% 25ppm/C 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 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 20102014 Analog Devices, Inc. All rights reserved. 09371-001CN-0179 Circuit Note Bypass capacitors (not shown in Figure 1) are required. In this CIRCUIT DESCRIPTION case, a 10 F tantalum capacitor in parallel with a 0.1 F ceramic For industrial and process control modules, 4 mA-to-20 mA capacitor should be placed on each power pin of each dual current loop transmitters are used as a means of communication op amp. Details of proper decoupling techniques can be between the control unit and the actuator. Located at the control found in Tutorial MT-101. unit, the 14-bit AD5641 DAC produces an output voltage, V , DAC between 0 V and 5 V as a function of the input code. The code Figure 2 shows the linearity of the system, that is the measured is set via an SPI interface. The ideal relationship between the output current from the circuit DAC input code from 0 to full-scale. input code and output voltage is given by 14 20 VDAC = VREF (D/2 ) (1) 18 where: 16 VREF is the output of ADR02 and the power supply to the AD5641. 14 D is the decimal equivalent of the binary code that is loaded to IDEAL CURRENT the AD5641. 12 MEASURED CURRENT 10 The DAC output voltage sets the current flowing through the sense resistor, RSENSE, where 8 6 ISENSE = VDAC/RSENSE (2) 4 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 2 voltage at the noninverting input of the AD8657 amplifier (A2). 0 The A2 AD8657 closes the loop and brings the inverting input voltage to the same voltage as the noninverting input. Therefore, DAC INPUT CODE the current flowing through R1 is mirrored by a factor of 10 to Figure 2. 0 mA to 20 mA Output Current R2. This is represented by Equation 3. Figure 3 shows the output current error plot in percent full-scale IOUT = IR2 = (VDAC/RSENSE ) ( R1/R2) (3) range. The overall worst-case error is approximately 0.35% With VDAC ranging from 0 V to 5 V, the circuit generates a measured over the output range between Code 256 and current output from 0 mA to 20 mA. Code 16,128. 0.35 The AD5641 is a 14-bit DAC from the nanoDAC family and operates from the 5 V output voltage of the ADR02 reference. It 0.30 has an on-chip precision output buffer that is capable of swinging 0.25 from rail-to-rail (within 10 mV), thus allowing a high dynamic output range. With a supply voltage of 5 V, AD5641 consumes 0.20 a typical 75 A of supply current. 0.15 In addition, this circuit solution requires a rail-to-rail input 0.10 amplifier. The AD8657 dual op amp is an excellent choice, with low power and rail-to-rail features. The op amp operates with a 0.05 typical supply current of 22 A/amplifier over the specified supply 0 voltage and input common-mode voltage. It also offers excellent noise and bandwidth per unit of current. The AD8657 is one of 0.05 the lowest power amplifiers that operate on supplies of up to 18 V. The ADR02 is an ultracompact, precision 5 V voltage reference. DAC INPUT CODE With an 18 V input voltage, quiescent current is only 650 A, Figure 3. Output Current Error Plot typical. It has an initial accuracy of 0.06% (B-grade) and 10 V p-p voltage noise. Connecting a 0.1 F ceramic capacitor to the output is highly recommended to improve stability and filter out low level voltage noise. An additional 1 F to 10 F electrolytic, tantalum, or ceramic capacitor in parallel can improve load transient response. A 1 F to 10 F 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 F ceramic capacitor should be connected in parallel to reduce supply noise. Rev. A Page 2 of 5 READING ERROR (%FSR) OUTPUT CURRENT (mA) 0 0 1024 1024 2048 2048 3072 3072 4096 4096 5120 5120 6144 6144 7168 7168 8192 8192 9216 9216 10240 10240 11264 11264 12288 12288 13312 13312 14336 14336 15360 15360 16384 16384 09371-002 09371-003