Circuit Note CN-0156 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/CN0281. Devices Connected/Referenced AD9834 75 MHz Complete Low Power DDS AD5620 12-Bit Voltage Output nanoDAC® Amplitude Control Circuit for AD9834 Waveform Generator (DDS) CIRCUIT FUNCTION AND BENEFITS or digital potentiometer is required to set the full-scale current. A voltage output DAC can be used to drive the FS ADJUST pin of the AD9834 through a series resistor. This determines the magnitude of the full-scale DAC current. The circuit shown in Figure 1 provides a simple approach for controlling the amplitude of the output waveform of an AD9834 75 MHz low power (20 mW) waveform generator (DDS). The DAC used in this example is the 12-bit AD5620, a member of the nanoDAC family. The AD5620 contains an on-chip 5 ppm/°C reference, has an SPI interface, and is available in an 8-lead SOT-23 or MSOP package. The low power (2.2 mW @ 3.3 V) and small size of the AD5620 (8-lead SOT-23) provide an attractive solution for generating an amplitude modulated output from the AD9834. DDS (direct digital synthesis) devices are capable of producing sine wave, square wave, and triangular output waveforms and, therefore, serve as waveform generators. Capability for phase modulation and frequency modulation is provided internally in the AD9834. However, in order to modulate the amplitude of the output signal, a low power DAC +3.3V VDD GND VFB AD5620 VDAC VOUT SYNC SCLK DIN RSET = 6.8kΩ +3.3V +3.3V AVDD DVDD FS ADJUST ADJUST PEAK-TO-PEAK USING AD5620 MICRO FSYNC SCLK IOUT 200Ω AD9834 IOUTB SDATA 200Ω MCLK 09019-001 DGND AGND Figure 1. Low Power Amplitude Control Circuit for AD9834 DDS (Simplified Schematic: All Connections and Decoupling Not Shown) 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 ©2010–2013 Analog Devices, Inc. All rights reserved. CN-0156 Circuit Note CIRCUIT DESCRIPTION The circuit operates on a 3 V to 5 V single supply. Both the DAC and DDS operate with an SPI interface. The on-chip DAC for many DDS devices provides a complementary current output, IOUT and IOUTB, for the AD9834 DDS. Increasing the voltage output from the AD5620 reduces the fullscale output current of the AD9834. The minimum full-scale current will be reached when the voltage from the AD5620 is equal to VREF, or 1.20 V. The reference current to the DAC is a function of the internal reference voltage, VREF, and an external resistor, RSET, which normally connects from the DAC FS ADJUST pin to ground. The reference current is equal to VREF/RSET where VREF is the internal reference of the AD9834 and has a typical value of 1.20 V. The RSET resistor has a typical value of 6.8 kΩ. Figure 3 shows the AD9834 output voltage for a half-scale output current, where VDAC = 0.5 × VREF, or 0.6 V. The full-scale current from the DAC is a multiple of the reference current. For example, the full-scale current of the AD9834 is I FULLSCALE = 18 × (VREF / RSET ) If FS ADJUST is connected to a varying voltage, VDAC, the fullscale current is 09019-003 I FULLSCALE = 18 × (VREF − VDAC ) / RSET Varying VDAC varies the full-scale current and, therefore, the voltage output from the DDS device. You can provide this varying voltage by using a voltage-output DAC. Figure 3. DDS Output When Amplitude Control DAC Is Set to 0.6 V (This Produces One-Half Scale DDS Output) The AD5620 is a low power, small, price effective solution here. A member of the nanoDAC family, it contains an on-chip 5 ppm/°C reference and is available in an 8-lead SOT-23 or MSOP package. The output voltage of the AD5620 is 0 V to +2.5 V. The circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see MT-031 Tutorial and MT-101 Tutorial). COMMON VARIATIONS The maximum full-scale output current is reached when VDAC = 0 V (zero-scale) and the current from the AD9834 swings between approximately 0.16 mA and 3.12 mA. With the 200 Ω load resistor, the AD9834 output voltage swings between approximately 0.032 V and 0.624 V. The output voltage on the IOUT pin of the AD9834 is shown in Figure 2, where the DDS output frequency is set for 1 MHz. The AD5640 and AD5660 are 14-bit and 16-bit versions, respectively, of the AD5620 that are suitable where higher resolutions are required. The AD9833 offers the same functionality of the AD9834 without the amplitude modulation capability. LEARN MORE ADIsimDDS Design and Evaluation Tool. Kester, Walt. The Data Conversion Handbook. 2005. Chapters 3, 7, and 8. 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. 09019-002 MT-085 Tutorial, Fundamentals of Direct Digital Synthesis (DDS). Analog Devices. Figure 2. DDS Output When Amplitude Control DAC Is Set to 0 V (This Produces Maximum DDS Full-Scale Output) MT-101 Tutorial, Decoupling Techniques. Analog Devices. Riordan, Liam. AN-1070 Application Note, Programming the AD9833/AD9834. Analog Devices. Rev. A | Page 2 of 3 Circuit Note CN-0156 Data Sheets and Evaluation Boards AD9834 Data Sheet AD9834 Evaluation Board AD5620 Data Sheet AD5620 Evaluation Board REVISION HISTORY 7/13—Rev. 0 to Rev. A Changes to Circuit Description Section and Figure 3 Caption ........ 2 6/10—Revision 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. ©2010–2013 Analog Devices, Inc. All rights reserved. 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