CN-0156: Amplitude Control Circuit for AD9834 Waveform Generator (DDS) PDF

Circuit Note
CN-0156
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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
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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
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CN09019-0-7/13(A)
Rev. A | Page 3 of 3
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