Circuit Note CN-0183 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/CN0183. AD5668 16-Bit Voltage Output denseDAC with 5 ppm/°C On-Chip Reference and SPI Interface AD8638 16 V Auto-Zero, Rail-to-Rail Output Op Amp ADP2300 1.2 A, 20 V, 700 kHz, Nonsynchronous StepDown Switching Regulator REF192 Precision Micropower, 2.5 V Low Dropout Voltage Reference Precision 16-Bit, Bipolar Output Voltage Source with +12 V to ±5 V Supply EVALUATION AND DESIGN SUPPORT of the AD8638 is only 0.06 ppm/°C. The external REF192 reference ensures a maximum drift of 5 ppm/°C (E grade) and provides a low impedance pseudo ground for the AD8638 level gain and shifting circuit. Circuit Evaluation Boards CN-0183 Circuit Evaluation Board (EVAL-CN0183-SDZ) System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials The circuit offers an efficient solution to a problem often encountered in systems with a single +12 V supply rail. Proper printed circuit board (PCB) layout and grounding techniques ensure that the ADP2300 switching regulator does not degrade the overall performance of the circuit. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 provides a precision 16-bit, low drift bipolar voltage output of ±2.5 V and operates on a single +10 V to +15 V supply. The unipolar voltage outputs of the AD5668 octal denseDAC are amplified and level shifted by the AD8638 auto-zero op amps. The maximum drift contribution +5V VIN = +12V REF192 +5V L1 100nF + 1µF + + ADP2300 BST VS +5V FB EN 0.1µF 10µF 1µF 10µF 10µF VIN +2.5V R1 10kΩ 1.5kΩ SW GND OUTPUT GND +5V 0.1µF 52.3kΩ 10kΩ + VDD + 4.7µF 10kΩ R2 10kΩ 10µF SYNC 1.5kΩ SCLK DIN VREFIN/VREFOUT TP1 DAC_OUT AD8638 DAC A VOUTA TP2 BIPOLAR_OUT −2.5V TO +2.5V 0V TO +2.5V –5V AD5668 –5V LDAC CLR DAC H GND VOUTH 09471-001 L1: COILCRAFT, LPD4012-472MLB, COUPLED INDUCTOR, 4.7µH Figure 1. Bipolar Output DAC Circuit with ±5 V Power Supplies Rev. A 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-0183 Circuit Note CIRCUIT DESCRIPTION 0.6 0.4 0.2 DNL (LSB) –0.2 –0.4 The output voltage of the AD5668 is 0 V to 2.5 V at TP1, and this signal drives the noninverting input of the AD8638 op amp. The signal gain of the op amp is 1 + R2/R1, which is 2 for R1 = R2. A negative 2.5 V offset is injected into the op amp output by driving R1 with the 2.5 V reference. The result is a bipolar output voltage at TP2 that swings from −2.5 V to +2.5 V. –0.8 0 10000 20000 30000 40000 DAC CODE 50000 60000 09471-003 –0.6 Figure 3. DNL Performance of Bipolar Output (TP2) 8 6 4 INL (LSB) The circuit operates on a single supply voltage of nominally 12 V, which can vary between 10 V and 15 V. The regulated −5 V supply rail is developed from an ADP2300 switching regulator connected in the inverting buck-boost configuration. The circuit can be designed using the ADIsimPower program available at www.analog.com/ADIsimPower. The L1 coupled inductor is used to develop an unregulated 5 V supply for the circuit using a Zeta configuration. This circuit yields high efficiency for small output currents. 0 2 0 –2 The integral nonlinearity (INL) and differential nonlinearity (DNL) measured at TP2 (bipolar output) are shown in Figure 2 and Figure 3, respectively. –4 –6 0 The INL and DNL measured at TP1 (unipolar DAC output) are shown in Figure 4 and Figure 5, respectively. 10000 20000 40000 30000 DAC CODE 50000 60000 09471-004 The AD5668 is a 16-bit, octal, voltage output denseDAC controlled by an SPI interface. It contains an on-chip reference with a 10 ppm/°C maximum drift specification. The on-chip reference is off at power-up, allowing the use of an external reference. The internal reference is enabled via a software write. In the circuit shown in Figure 1, an external REF192 is used because a low output impedance is required to drive the 2.5 V pseudo ground reference for the AD8638 op amps. Figure 4. INL Performance of Unipolar DAC Output (TP1) 0.8 6 0.6 5 0.4 3 0.2 DNL (LSB) 4 1 0 0 –0.2 –0.4 –1 –2 –0.6 –3 –0.8 –1.0 –5 0 10000 20000 30000 40000 DAC CODE 50000 60000 0 10000 20000 40000 30000 DAC CODE 50000 60000 Figure 5. DNL Performance of Unipolar DAC Output (TP1) Figure 2. INL Performance of Bipolar Output (TP2) Rev. A | Page 2 of 5 09471-005 –4 09471-002 INL (LSB) 2 Circuit Note CN-0183 COMMON VARIATIONS Getting Started The AD5628 and AD5648 are 12-bit and 14-bit versions of the AD5668. All have an on-chip reference with an internal gain of 2. The AD5628-1/AD5648-1/AD5668-1 have a 1.25 V, 5 ppm/°C reference, giving a full-scale output range of 2.5 V; and the AD5628-2/AD5648-2/AD5668-2 and AD5668-3 have a 2.5 V, 5 ppm/°C reference, giving a full-scale output range of 5 V. The on-board reference is off at power-up, allowing the use of an external reference. The internal reference is enabled via a software write. The part incorporates a power-on-reset circuit that ensures that the DAC output powers up to 0 V (AD5628-1/AD5648-1/ AD5668-1, AD5628-2/AD5648-2/AD5668-2) or midscale (AD5668-3) and remains powered up at this level until a valid write takes place. Load the evaluation software by placing the CN-0183 evaluation software CD in the CD drive of the PC. Using My Computer, locate the drive that contains the evaluation software CD and open the Readme file. Follow the instructions contained in the Readme file for installing and using the evaluation software. The evaluation software main window is shown in Figure 6. 09471-006 The AD8639 is a dual version of the AD8638 and can be used, if desired. The circuit in Figure 1 uses the single AD8638 to minimize crosstalk between the eight channels. Other 2.5 V references can be used, such as the ADR4525, which has an accuracy of ±0.02% and a temperature coefficient of 2 ppm/°C maximum (B grade). Figure 6. Evaluation Software Main Window Functional Diagram of Test Setup A functional diagram of the test setup is shown in Figure 7. This setup allows the DAC output (TP1) and the bipolar output (TP2) to be observed with an oscilloscope. CIRCUIT EVALUATION AND TEST Equipment Needed (Equivalents Can Be Substituted) The following equipment is needed: • The System Demonstration Platform (EVAL-SDP-CB1Z) • The CN-0183 circuit evaluation board (EVAL-CN0183-SDZ) • The CN-0183 evaluation software • The Tektronix TDS2024, 4-channel oscilloscope • The HP E3630A 0 V to 6 V/2.55 A; ±20 V/0.5 A triple output dc power supply • A PC (Windows 32-bit or 64-bit) Linearity measurements require a precision, digital voltmeter (DVM) that can be read by the PC via a USB port. POWER SUPPLY +5V GND –5V +12V OSCILLOSCOPE GND PC USB AVDD AGND J5-2 –5V +12V AGND J5-3 J4-1 USB J4-2 DAC_OUT/TP1 BIPOLAR_OUT/TP2 EVAL-CN0183-SDZ SDP CON A OR CON B 09471-007 J1 120-PIN SDP J5-1 Figure 7. Functional Diagram of Test Setup Rev. A | Page 3 of 5 CN-0183 Circuit Note Setup Connect the 120-pin connector on the EVAL-CN0183-SDZ to the CON A connector or the CON B connector on the EVALSDP-CB1Z. Use nylon hardware to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors. After successfully setting the dc output supply to +5 V, −5 V, and +12 V, turn the power supply off. With power to the supply off, connect the −5 V power supply to the −5V pin on J5-3, connect the +5 V power supply to the AVDD pin on J5-1, connect GND to the AGND pins on J5-2 and J4-2, and connect the +12 V power supply to the +12V pin on J4-1. Alternatively, place Link 2 and Link 3 in Position B to power the circuitry using the ADP2300 to supply +5 V and −5 V. Note that AVDD and the −5 V are not needed in this case. Turn on the power supply and then connect the USB cable from the SDP board to the USB port on the PC. Do not connect the USB cable to the mini-USB connector on the SDP before turning on the dc power supply for the EVAL-CN0183-SDZ. After setting up the test equipment, connect the probes of the oscilloscope to the TP1 and TP2 test points. The TP3, TP4, and TP5 test points are connected to the reference, the regulated +5 V, and the regulated −5 V, respectively. Check these test points for the correct voltages (use TP6 for the ground). The software provided on the CD allows users to set the value of VOUTA by loading a code into the DAC and by choosing the source of the reference. If users keep the default setting, they will have to supply the +5 V and −5 V voltages, and the +12 V is not required. The default setting uses the external REF192 reference, giving you a full-scale DAC output range of 2.5 V (TP1), and −2.5 V to +2.5 V on the bipolar output (TP2). Loading 0x0000 sets the DAC output to 0 V and the bipolar output to −2.5 V. Loading 0x8000 sets the DAC output to 1.25 V and the bipolar output to 0 V. Loading 0xFFFF sets the DAC output to 2.5 V and the bipolar output to 2.5 V. Table 1. Jumper Settings for EVAL-CN0183-SDZ (Default Settings in Bold) Jumper LK1 Description Short AD5668 reference pin to REF192 output LK2 AVDD supply source Setting Inserted Opened Position A Position B LK5 −5 V voltage source Position A Position B Function It shorts AD5668 reference pin to REF192 output allowing the use of an external DAC reference. Only the internal reference of the AD5668 can be used. The circuit is powered by an external 5 V supply applied to the AVDD pin on J5-1. The digital power is supplied by the 5 V voltage supplied by the ADP2300 regulator. The analog circuit is supplied by an external power supply apply to the −5V pin on J5-3. The digital power is supplied by the −5 V voltage obtained by inverting the output of the ADP2300 regulator. Rev. A | Page 4 of 5 Circuit Note CN-0183 LEARN MORE Data Sheets and Evaluation Boards CN-0183 Design Support Package: http://www.analog.com/CN0183-DesignSupport CN-0183 Circuit Evaluation Board (EVAL-CN0183-SDZ) Ardizzoni, John. A Practical Guide to High-Speed PrintedCircuit-Board Layout, Analog Dialogue 39-09, September 2005. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of “AGND” and “DGND”, Analog Devices. System Demonstration Platform (EVAL-SDP-CB1Z) AD5668 Data Sheet and Evaluation Board AD8638 Data Sheet and Evaluation Board ADP2300 Data Sheet and Evaluation Board REF192 Data Sheet and Evaluation Board MT-101 Tutorial, Decoupling Techniques, Analog Devices. REVISION HISTORY ADIsimPower Design Tool 6/13—Rev. 0 to Rev. A Change to Figure 1 ............................................................................ 1 6/12—Rev. 0: Initial Version (Continued from first page) Circuits from the Lab circuits 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 circuits 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 circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits 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 circuits at any time without notice but is under no obligation to do so. ©2012–2013 Analog Devices, Inc. All rights reserved. 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