Circuit Note CN-0200 Devices Connected/Referenced AD5780 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/CN0200. True 18-Bit, Voltage Output DAC Ultraprecision, 36 V, 2.8 nV√Hz, Rail-to-Rail Output Op Amp Ultraprecision, 36 V, 2.8 nV√Hz, Dual Rail-to-Rail Output Op Amp Ultralow Noise 5V LDO XFET® Voltage Reference AD8675 AD8676 ADR445 18-Bit, Linear, Low Noise, Precision Bipolar ±10 V DC Voltage Source EVALUATION AND DESIGN SUPPORT amount of external components. The AD5780 DAC is an 18-bit, unbuffered voltage output DAC operating from a bipolar supply of up to 33 V. The AD5780 accepts a positive reference input range of 5 V to VDD − 2.5 V, and a negative reference input range of VSS + 2.5 V to 0 V. Both reference inputs are buffered on the chip, and external buffers are not required. The AD5780 offers a relative accuracy specification of ±1 LSB maximum, and operation is guaranteed monotonic, with a ±1 LSB maximum DNL specification. Circuit Evaluation Boards AD5780 Circuit Evaluation Board (EVAL-AD5780SDZ) System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is an 18-bit linear, low noise, precision bipolar (±10 V) voltage source with a minimum +15V 10µF + +15V 0.1µF VIN VOUT ADR445 GND R1 1.5kΩ 0.1µF + C1 10µF AD8675 +3.3V +15V R2* R3* 1kΩ +10V A1 1kΩ –15V 10µF + 10µF + 0.1µF 0.1µF 9 SPI INTERFACE AND DIGITAL COTNROL 8 3 2 IOVCC VCC VDD 6 CLR 7 LDAC 11 SDO 14 SYNC 13 SCLK 12 SDIN 4 RESET VREFP 20 RFB BUFFERED VREFP 6.8kΩ +15V 6.8kΩ INV AD8675 24 A2 VOUT 1 AD5780 –10V TO +10V OUTPUT VOLTAGE –15V DGND VSS VREFN AGND 15 17 16 19 0V TO +10V 0.1µF + 09697-001 *FOR OPTIMUM PERFORMANCE OVER TEMPERATURE, R2 AND R3 SHOULD BE IN A SINGLE PACKAGE. 10µF –15V Figure 1. 18-Bit Accurate, +10 V Voltage Source (Simplified Schematic: All Connections and Decoupling Not Shown) Rev.0 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 ©2011 Analog Devices, Inc. All rights reserved. CN-0200 Circuit Note This combination of parts provides industry-leading 18-bit integral nonlinearity (INL) of ±1 LSB and differential nonlinearity (DNL) of ±0.75 LSB, with guaranteed monotonicity, as well as low power, small PCB area, and cost effectiveness in an LFCSP package. 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 0 50000 100000 150000 200000 250000 DAC CODE 09697-002 The digital input to the circuit is serial and is compatible with standard SPI, QSPI, MICROWIRE®, and DSP interface standards. For high accuracy applications, the compact circuit offers high precision, as well as low noise—this is ensured by the combination of the AD5780, ADR445, and AD8675 precision components. 0.6 INL (LSB) The AD8675 precision op amp has low offset voltage (75 µV maximum), low noise (2.8 nV/√Hz typical), and is an optimum output buffer for the AD5780. The AD5780 has two internal matched feedforward and feedback resistors, which are connected to the AD8675 op amp and provide the 10 V offset voltage. This allows an output voltage swing of ±10 V with a single external 10 V reference. Figure 2. Integral Nonlinearity vs. DAC Code Figure 3 shows that the differential nonlinearity as a function of DAC code is within the −0.25 LSB to +0.75 LSB specification. The digital-to-analog converter (DAC) shown in Figure 1 is the AD5780, a high voltage, 18-bit converter with SPI interface, offering ±1 LSB INL, ±0.75 LSB DNL, and 7.5 nV/√Hz noise spectral density. The AD5780 also exhibits an extremely low temperature drift of 0.005 LSB/°C. Linearity Measurements The precision performance of the circuit shown in Figure 1 is demonstrated on the EVAL-AD5780SDZ evaluation board using an Agilent 3458A Multimeter. Figure 2 shows the integral nonlinearity as a function of DAC code is within specifications of ±1 LSB. 0.4 0.3 DNL (LSB) Figure 1 shows the AD5780 configured in a gain-of-two mode such that a single reference source can be used to generate a symmetrical bipolar output voltage range. This mode of operation uses an external op amp (A2), as well as on-chip resistors (see AD5780 data sheet), to provide the gain of 2. These internal resistors are thermally matched to each other and to the DAC ladder resistance, resulting in ratiometric thermal tracking. The output buffer is again the AD8675, used for its low noise and low drift. This amplifier is also used (A1) to amplify the +5 V reference voltage from the low noise ADR445 to +10 V. R2 and R3 in this gain circuit are precision metal foil resistors with 0.01% tolerance and a temperature coefficient resistance of 0.6 ppm/°C. For optimum performance over temperature, R1 and R2 should be in a single package, such as the Vishay 300144 or VSR144 series. R2 and R3 are selected to be 1 kΩ to keep noise in the system low. R1 and C1 form a low-pass filter with a cutoff frequency of approximately 10 Hz. The purpose of this filter is to attenuate voltage reference noise. 0.5 0.2 0.1 0 –0.1 –0.2 0 50000 100000 150000 200000 DAC CODE 250000 09697-003 CIRCUIT DESCRIPTION Figure 3. Differential Nonlinearity vs. DAC Code Noise Drift Measurements To be able to realize high precision, the peak-to-peak noise at the circuit output must be maintained below 1 LSB, which is 76.29 µV for 18-bit resolution and a 20 V peak-to-peak voltage range. A real-time noise application will not have a high-pass cutoff at 0.1 Hz to attenuate 1/f noise but will include frequencies down to dc in its pass band. With this in mind, the measured peak-topeak noise is realistically shown in Figure 4. In this case, the noise at the output of the circuit was measured over a period of 100 seconds, effectively including frequencies as low as 0.01 Hz in the measurement. The upper frequency cutoff is at approximately 14 Hz and is limited by the measurement setup. Rev. 0 | Page 2 of 6 Circuit Note CN-0200 4 Figure 4 shows the peak-to-peak values are 1.2 µV for zero-scale output, 32 µV for half-scale output, and 64 µV for full-scale output. 3 40 2 NOISE (µV) The zero-scale output voltage exhibits the lowest noise because it represents the noise from the DAC core only. The noise contribution from each voltage reference path is attenuated by the DAC when the zero-scale code is selected. FULL SCALE 0 –3 10 ZERO SCALE 0 20 40 60 80 100 TIME (Seconds) 0 Figure 5. DAC Output Voltage Noise Measured Over 100 Second Period for Full Scale (Red), Half Scale (Green), and Zero Scale (Blue) with Precision Reference Source –10 HALF SCALE –20 VDD = +15V VSS = –15V VREFP = +10V VREFN = 0V –30 –40 HALF SCALE 0 20 40 60 09697-005 –2 ZERO SCALE 80 TIME (Seconds) COMMON VARIATIONS 100 09697-004 NOISE (µV) 1 –1 30 20 FULL SCALE VDD = +15V VSS = –15V VREFP = +10V VREFN = 0V Figure 4. DAC Output Voltage Noise Measured Over 100 Second Period for Full Scale (Red), Half Scale (Green), and Zero Scale (Blue) As the time period over which the measurement is taken is increased, lower frequencies will be included, and the peak-topeak value will increase. At low frequencies, temperature drift and thermocouple effects become contributors to noise. These effects can be minimized by choosing components with low thermal coefficients. In this circuit, the main contributor to low frequency 1/f noise is the voltage reference. It also exhibits the greatest temperature coefficient value in the circuit of 3 ppm/°C. A temperature controlled ultralow noise reference would be required to improve the half-scale and full-scale DAC output noise. The AD5780 will support a wide variety of output ranges from 0 V to +5 V up to ±10 V, and values in between. The gain-of-2 configuration, as shown in Figure 1, can be used if a symmetrical output range is required. This mode is selected by setting the RBUF bit of the AD5780 internal control register to a Logic 0. If an asymmetrical range is required, individual references can be applied at VREFP and VREFN, and the output buffer should be configured for unity gain as described in the AD5780 data sheet. This is done by setting the RBUF bit of the AD5780 internal control register to a Logic 1. The AD8676 is a dual version of the AD8675 op amp and can be used in the circuit if desired. CIRCUIT EVALUATION AND TEST Equipment Required Figure 5 shows the performance of the signal chain by replacing the ADR445 with a Krohn Hite Model 523 Precision Reference set for +5 V. Complete schematics and layout of the printed circuit board can be found in the CN-0200 Design Support Package: www.analog.com/CN0200-DesignSupport . • System Demonstration Platform (EVAL-SDP-CB1Z) • EVAL-AD5780SDZ Evaluation Board and Software • Agilent 3458A multimeter • PC (Windows 32-bit or 64-bit) • National Instruments GPIB to USB-B interface cable • SMB cable (1) Software Installation The AD5780 evaluation kit includes self-installing software on a CD. The software is compatible with Windows XP (SP2) and Vista (32-bit and 64-bit). If the setup file does not run automatically, you can run the setup.exe file from the CD. Rev. 0 | Page 3 of 6 CN-0200 Circuit Note Install the evaluation software before connecting the evaluation board and SDP board to the USB port of the PC to ensure that the evaluation system is correctly recognized when connected to the PC. After installation from the CD is complete, power up the AD5780 evaluation board as described in the Power Supplies section of UG-256. Connect the SDP board (via either Connector A or Connector B) to the AD5780 evaluation board and then to the USB port of your PC using the supplied cable. 2. When the evaluation system is detected, proceed through any dialog boxes that appear. This completes the installation. A functional diagram of the test setup is shown in Figure 7. Power Supply Configuration The following supplies must be provided: • • • 3.3 V between the VCC and DGND on Connector J1 for the digital supply of the AD5780. Alternatively, place Link 1 in Position A to power the digital circuitry from the USB port via the SDP board (default setting). +12 V to +16.5 V between the VDD and AGND inputs of J2 for the positive analog supply of the AD5780. −12 V to −16.5 V between the VSS and AGND inputs of J2 for the negative analog supply of the AD5780. 09697-006 1. Functional Diagram Figure 6. Evaluation Software Main Window Rev. 0 | Page 4 of 6 Circuit Note CN-0200 GPIB DUAL POWER SUPPLY COM +15V AGILENT 3458A MULTIMETER USB PC USB −15V +VDD AGND −VSS J2-3 J2-2 J2-1 USB SMB VOUT_BUF SDP CON A OR CON B 09697-007 J4 120-PIN SDP EVAL-AD5780SDZ Figure 7. Functional Block Diagram of Test Setup Link Configuration Setup Test The default link options are listed in Table 1. By default, the board is configured with VREFP = +10 V and VREFN = −10 V for a ±10 V output range. The VOUT_BUF SMB connector is connected to the Agilent 3458A multimeter. The linearity measurements are run using the Measure DAC Output Tab on the AD5780 GUI. Table 1. Default Link Options The noise drift measurement is measured on the VOUT_BUF SMB connector also. The output voltage is set using the Prgram Voltage tab in the AD5780 GUI. The peak-to-peak noise drift is measured over 100 seconds. Link No. LK1 LK2 LK3 LK4 LK5 LK6 LK7 LK8 LK9 LK11 Option A B A Removed Removed Removed Removed C Inserted Inserted For more details on the definitions and how to calculate the INL, DNL, and noise from the measured data, see the "TERMINOLOGY" section of the AD5780 data sheet and also the following reference: Data Conversion Handbook, "Testing Data Converters," Chapter 5, Analog Devices. LEARN MORE CN-0200 Design Support Package: www.analog.com/CN0200-DesignSupport To configure the board for the circuit shown in Figure 1, the following changes must be made to the default link configuration in Table 1: 1. 2. 3. Egan, Maurice. "The 20-Bit DAC Is the Easiest Part of a 1-ppmAccurate Precision Voltage Source," Analog Dialogue, Vol. 44, April 2010. Place LK3 in position B. Insert LK4. Place LK8 in position B. Kester, Walt. 2005. The Data Conversion Handbook. Analog Devices. Chapters 3, 5, and 7. These changes configure the output buffer amplifier for a gain of 2 and connect the VREFN pin of the AD5780 to ground. MT-015 Tutorial, Basic DAC Architectures II: Binary DACs. Analog Devices. Please refer to User Guide UG-256 for more information on the EVAL-AD5780SDZ test setup. MT-016 Tutorial, Basic DAC Architectures III: Segmented DACs. Analog Devices. Rev. 0 | Page 5 of 6 CN-0200 MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of AGND and DGND. Analog Devices. MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues. Analog Devices. Circuit Note REVISION HISTORY 11/11—Revision 0: Initial Version MT-101 Tutorial, Decoupling Techniques. Analog Devices. Voltage Reference Wizard Design Tool. CN-0177 Circuit Note, 18-Bit, Linear, Low Noise, Precision Bipolar ±10 V DC Voltage Source. CN-0191 Circuit Note, 20-Bit, Linear, Low Noise, Precision, Bipolar ±10 V DC Voltage Source. User Guide UG-256 for EVAL-AD5780SDZ. Data Sheets and Evaluation Boards AD5780 Data Sheet and Evaluation Board AD8676 Data Sheet ADR445 Data Sheet (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" 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. ©2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN09697-0-11/11(0) Rev. 0 | Page 6 of 6