11 mW Power, 2.3 V to 5.5 V,
FEATURES
2.3 V to 5.5 V power supply
MCLK speed: 16 MHz (B grade), 5 MHz (A grade)
Output frequency up to 8 MHz
Sinusoidal and triangular outputs
On-board comparator
3-wire SPI interface
Extended temperature range: −40°C to +125°C
Power-down option
11 mW power consumption at 2.3 V
20-lead LFCSP
APPLICATIONS
Frequency stimulus/waveform generation
Frequency phase tuning and modulation
Low power RF/communications systems
Liquid and gas flow measurement
Sensory applications: proximity, motion, and defect detection
Test and medical equipment
GENERAL DESCRIPTION
The AD9838 is a low power DDS device capable of producing high
performance sine and triangular outputs. It also has an on-board
comparator that allows a square wave to be produced for clock
generation. Consuming only 11 mW of power at 2.3 V, the
AD9838 is an ideal candidate for power-sensitive applications.
FUNCTIONAL BLOCK DIAGRAM
DGND
CAP/2.5VDVDDAGNDAVDD
Complete DDS
AD9838
Capability for phase modulation and frequency modulation is
provided. The frequency registers are 28 bits wide: with a 16 MHz
clock rate, resolution of 0.06 Hz can be achieved; with a 5 MHz
clock rate, the AD9838 can be tuned to 0.02 Hz resolution.
Frequency and phase modulation are configured by loading
registers through the serial interface and by toggling the registers
using software or the FSELECT and PSELECT pins, respectively.
The AD9838 is written to via a 3-wire serial interface. This serial
interface operates at clock rates up to 40 MHz and is compatible
with DSP and microcontroller standards.
The device operates with a power supply from 2.3 V to 5.5 V. The
analog and digital sections are independent and can be run from
different power supplies; for example, AVDD can equal 5 V with
DVDD equal to 3 V.
The AD9838 has a power-down pin (SLEEP) that allows external
control of the power-down mode. Sections of the device that are
not being used can be powered down to minimize current consumption. For example, the DAC can be powered down when
a clock output is being generated.
The AD9838 is available in a 20-lead LFCSP_WQ package.
REFOUT FSADJUST
MCLK
FSELECT
28-BIT FREQ0
REG
28-BIT FREQ1
REG
SERIAL INT ERFACE
CONTROL L OGIC
FSYNC SCLK SDATA
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Anal og Devices for its use, nor for any infringements of patents or ot her
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
MUX
12-BIT PHASE0 REG
12-BIT PHASE1 REG
AND
REGULATOR
PHASE
ACCUMULATOR
(28-BIT)
16-BIT CONTRO L
REGIS TER
Σ
MUX
VCC
2.5V
12
MUX
MUX
Figure 1.
ON-BOARD
REFERENCE
SIN
ROM
SLEEP RESETPSELECT
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.
DIVIDE
BY 2
MUX
MSB
FULL-SCALE
CONTROL
10-BIT
DAC
COMPARATOR
AD9838
COMP
IOUT
IOUTB
SIGN BIT OUT
VIN
09077-001
AD9838
TABLE OF CONTENTS
Features.............................................................................................. 1
Applications....................................................................................... 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Timing Characteristics ................................................................ 5
Absolute Maximum Ratings............................................................ 6
Thermal Resistance ...................................................................... 6
ESD Caution.................................................................................. 6
Pin Configuration and Function Descriptions............................. 7
Typical Performance Characteristics ............................................. 9
Test Circuit ...................................................................................... 12
Terminology .................................................................................... 13
Theory of Operation ...................................................................... 14
Circuit Description......................................................................... 15
Numerically Controlled Oscillator Plus Phase Modulator... 15
SIN ROM..................................................................................... 15
Digital-to-Analog Converter (DAC) ....................................... 15
Comparator................................................................................. 15
Regulator...................................................................................... 16
Functional Description.................................................................. 17
Serial Interface............................................................................ 17
Latency Period ............................................................................ 17
Control Register ......................................................................... 17
Frequency and Phase Registers ................................................ 19
Reset Function ............................................................................ 20
Sleep Function ............................................................................ 20
SIGN BIT OUT Pin.................................................................... 21
IOUT and IOUTB Pins ............................................................. 21
Powering Up the AD9838 ......................................................... 21
Applications Information.............................................................. 24
Grounding and Layout.............................................................. 24
Interfacing to Microprocessors................................................. 24
Evaluation Board............................................................................ 26
System Demonstration Platform.............................................. 26
AD9838 to SPORT Interface..................................................... 26
Evaluation Kit ............................................................................. 26
Crystal Oscillator vs. External Clock....................................... 26
Power Supply............................................................................... 26
Evaluation Board Schematics ................................................... 27
Evaluation Board Layout........................................................... 29
Outline Dimensions....................................................................... 30
Ordering Guide .......................................................................... 30
REVISION HISTORY
4/11—Rev. 0 to Rev. A
Change to Title.................................................................................. 1
Change to Figure 3 ........................................................................... 5
Change to Figure 8 ........................................................................... 9
4/11—Revision 0: Initial Version
Rev. A | Page 2 of 32
AD9838
SPECIFICATIONS
AVDD = DVDD = 2.3 V to 5.5 V, AGND = DGND = 0 V, TA = T
otherwise noted.
Table 1.
Parameter1 Min Typ Max Unit Test Conditions/Comments
SIGNAL DAC SPECIFICATIONS
Resolution 10 Bits
Update Rate
A Grade 5 MSPS
B Grade 16 MSPS
I
Full Scale2 3.0 mA
OUT
V
Maximum 0.6 V
OUT
V
Minimum 30 mV
OUT
Output Compliance3 0.8 V
DC Accuracy
Integral Nonlinearity (INL) ±1 LSB
Differential Nonlinearity (DNL) ±0.5 LSB
DDS SPECIFICATIONS
Dynamic Specifications
Signal-to-Noise Ratio (SNR)
A Grade −63 dB f
B Grade −64 dB f
Total Harmonic Distortion (THD)
A Grade −64 dBc f
B Grade −64 dBc f
Spurious-Free Dynamic Range (SFDR)
Wideband (0 to Nyquist)
A Grade −68 dBc f
B Grade −66 dBc f
Narrow-Band (±200 kHz)
A Grade −97 dBc f
B Grade −92 dBc f
Clock Feedthrough
A Grade −68 dBc f
B Grade −65 dBc f
Wake-Up Time 1 ms
COMPARATOR
Input Voltage Range 1 V p-p AC-coupled internally
Input Capacitance 10 pF
Input High-Pass Cutoff Frequency 3 MHz
Input DC Resistance 5 MΩ
Input Leakage Current 10 μA
OUTPUT BUFFER
Output Rise/Fall Time 12 ns Using a 15 pF load
Output Jitter 120 ps rms 3 MHz sine wave 0.6 V p-p
VOLTAGE REFERENCE
Internal Reference 1.11 1.18 1.24 V
REFOUT Output Impedance4 1 kΩ
Reference TC 100 ppm/°C
FSADJUST Voltage 1.14 V
MIN
to T
MAX
, R
SET
= 6.8 kΩ, R
= 200 Ω for IOUT and IOUTB, unless
LOAD
= 5 MHz, f
MCLK
= 16 MHz, f
MCLK
= 5 MHz, f
MCLK
= 16 MHz, f
MCLK
= 5 MHz, f
MCLK
= 16 MHz, f
MCLK
= 5 MHz, f
MCLK
= 16 MHz, f
MCLK
= 5 MHz, f
MCLK
= 16 MHz, f
MCLK
= f
OUT
OUT
= f
OUT
OUT
= f
OUT
OUT
= f
OUT
OUT
= reset
OUT
OUT
/4096
MCLK
= f
MCLK
/4096
MCLK
= f
MCLK
/50
MCLK
= f
MCLK
/50
MCLK
= f
MCLK
= reset
/4096
/4096
/50
/50
Rev. A | Page 3 of 32
AD9838
Parameter1 Min Typ Max Unit Test Conditions/Comments
LOGIC INPUTS
Input High Voltage, V
2.0 V 2.7 V to 3.6 V power supply
2.8 V 4.5 V to 5.5 V power supply
Input Low Voltage, V
0.7 V 2.7 V to 3.6 V power supply
0.8 V 4.5 V to 5.5 V power supply
Input Current, I
INH/IINL
Input Capacitance, CIN 3 pF
POWER SUPPLIES
AVDD 2.3 5.5 V f
DVDD 2.3 5.5 V
5
I
3.7 5 mA
AA
5
I
I
DD
A Grade 0.9 2 mA
B Grade 1.2 2.4 mA
5
IAA + I
See Figure 6
DD
A Grade 4.6 7 mA
B Grade 4.9 7.4 mA
Low Power Sleep Mode DAC powered down; see Table 17
A Grade 0.4 mA
B Grade 0.4 mA
1
Operating temperature range is −40°C to +125°C; typical specifications are at 25°C.
2
For compliance with the specified load of 200 Ω, I
3
Guaranteed by design.
4
Applies when REFOUT is sourcing current. The impedance is higher when REFOUT is sinking current.
5
Measured with the digital inputs static and equal to 0 V or DVDD.
1.7 V 2.3 V to 2.7 V power supply
INH
0.6 V 2.3 V to 2.7 V power supply
INL
10 μA
full scale should not exceed 4 mA.
OUT
= 16 MHz, f
MCLK
code dependent; see Figure 7
DD
OUT
= f
MCLK
/4096
Rev. A | Page 4 of 32
AD9838
TIMING CHARACTERISTICS
DVDD = 2.3 V to 5.5 V, AGND = DGND = 0 V, unless otherwise noted.
Table 2.
Parameter1 Limit at T
t1 200/62.5 ns min MCLK period (5 MHz/16 MHz)
t2 80/26 ns min MCLK high duration (5 MHz/16 MHz)
t3 80/26 ns min MCLK low duration (5 MHz/16 MHz)
t4 25 ns min SCLK period
t5 10 ns min SCLK high duration
t6 10 ns min SCLK low duration
t7 5 ns min FSYNC to SCLK falling edge setup time
t8 10 ns min SCLK falling edge to FSYNC rising edge time
t
− 5 ns max
4
t9 5 ns min Data setup time
t10 3 ns min Data hold time
t11 8 ns min FSELECT, PSELECT setup time before MCLK rising edge
t
8 ns min FSELECT, PSELECT setup time after MCLK rising edge
11A
t12 5 ns min SCLK high to FSYNC falling edge setup time
1
Guaranteed by design; not production tested.
Timing Diagrams
MIN
to T
Unit Description
MAX
MCLK
t
2
Figure 2. Master Clock
t
1
t
3
09077-003
MCLK
t
11A
09077-004
FSELECT,
PSELECT
t
11
VALID DATA VALID DATA VALID DATA
Figure 3. Control Timing
SCLK
FSYNC
SDATA
t
12
t
7
D15 D14 D2 D1 D15 D14
t
5
t
6
Figure 4. Serial Timing
t
4
t
8
t
10
t
9
D0
09077-005
Rev. A | Page 5 of 32
AD9838
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 3.
Parameter Rating
AVDD to AGND −0.3 V to +6 V
DVDD to DGND −0.3 V to +6 V
AVDD to DVDD −0.3 V to +0.3 V
AGND to DGND −0.3 V to +0.3 V
CAP/2.5V 2.75 V
Digital I/O Voltage to DGND −0.3 V to DVDD + 0.3 V
Analog I/O Voltage to AGND −0.3 V to AVDD + 0.3 V
Operating Temperature Range
Industrial (B Version) −40°C to +125°C
Storage Temperature Range −65°C to +150°C
Maximum Junction Temperature 150°C
Lead Temperature, Soldering (10 sec) 300°C
IR Reflow, Peak Temperature 220°C
Reflow Soldering (Pb Free)
Peak Temperature 260°C (+0/−5)
Time at Peak Temperature 10 sec to 40 sec
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
THERMAL RESISTANCE
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
Table 4. Thermal Resistance
Package Type θJA θJC Unit
20-Lead LFCSP_WQ (CP-20-10) 49.5 5.3 °C/W
ESD CAUTION
Rev. A | Page 6 of 32
AD9838
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
T
UST
FSADJ
IOUTB
IOUT
19 REFO U
20 COMP
16
18
17
1
AVD D
2
DVDD
CAP/2.5V
DGND
MCLK
NOTES
1. CONNECT EXP OSED PAD TO GROUND.
3
4
5
AD9838
TOP
VIEW
(Not to Scale)
8
7
6
RESET
FSELECT
PSELECT
15 AGND
VIN
14
13
SIGN BIT OUT
12
FSYNC
11
SCLK
9
10
TA
SLEEP
SDA
09077-006
Figure 5. Pin Configuration
Table 5. Pin Function Descriptions
Pin No. Mnemonic Description
1 AVDD
Positive Power Supply for the Analog Section. AVDD can have a value from 2.3 V to 5.5 V. A 0.1 μF decoupling
capacitor should be connected between AVDD and AGND.
2 DVDD
Positive Power Supply for the Digital Section. DVDD can have a value from 2.3 V to 5.5 V. A 0.1 μF decoupling
capacitor should be connected between DVDD and DGND.
3 CAP/2.5V
The digital circuitry operates from a 2.5 V power supply. This 2.5 V is generated from DVDD using an on-board
regulator when DVDD exceeds 2.7 V. The regulator requires a decoupling capacitor of 100 nF typical, which is
connected from CAP/2.5V to DGND. If DVDD is less than or equal to 2.7 V, CAP/2.5V should be shorted to DVDD.
4 DGND Digital Ground.
5 MCLK
Digital Clock Input. DDS output frequencies are expressed as a binary fraction of the frequency of MCLK. The
output frequency accuracy and phase noise are determined by this clock.
6 FSELECT
Frequency Select Input. FSELECT controls which frequency register, FREQ0 or FREQ1, is used in the phase
accumulator. The frequency register to be used can be selected using the FSELECT pin or the FSEL bit. When
the FSEL bit is used to select the frequency register, the FSELECT pin should be tied to CMOS high or low.
7 PSELECT
Phase Select Input. PSELECT controls which phase register, PHASE0 or PHASE1, is added to the phase accumulator
output. The phase register to be used can be selected using the PSELECT pin or the PSEL bit. When the PSEL bit
is used to select the phase register, the PSELECT pin should be tied to CMOS high or low.
8 RESET
Active High Digital Input. This pin resets the appropriate internal registers to 0 (this corresponds to an analog
output of midscale). RESET does not affect any of the addressable registers.
9 SLEEP
Active High Digital Input. When this pin is high, the DAC is powered down. This pin has the same function as
the SLEEP12 control bit.
10 SDATA Serial Data Input. The 16-bit serial data-word is applied to this input.
11 SCLK Serial Clock Input. Data is clocked into the AD9838 on each falling edge of SCLK.
12 FSYNC
Active Low Control Input. FSYNC is the frame synchronization signal for the input data. When FSYNC is taken
low, the internal logic is informed that a new word is being loaded into the device.
13 SIGN BIT OUT
Logic Output. The comparator output is available on this pin or, alternatively, the MSB from the NCO can be
output on this pin. Setting the OPBITEN bit in the control register to 1 enables this output pin. The SIGN/PIB bit
determines whether the comparator output or the MSB from the NCO is output on this pin.
14 VIN
Input to Comparator. The comparator can be used to generate a square wave from the sinusoidal DAC output.
The DAC output should be filtered appropriately before it is applied to the comparator to reduce jitter. When
the OPBITEN and SIGN/PIB bits in the control register are set to 1, the comparator input is connected to VIN.
15 AGND Analog Ground.
16, 17
IOUT,
IOUTB
Current Output. This is a high impedance current source. A load resistor of nominally 200 Ω should be connected
between IOUT and AGND. IOUTB should be tied to AGND through an external load resistor of 200 Ω, but it can
be tied directly to AGND. A 20 pF capacitor to AGND is also recommended to prevent clock feedthrough.
Rev. A | Page 7 of 32
AD9838
Pin No. Mnemonic Description
18 FSADJUST
Full-Scale Adjust Control. A resistor (R
of the full-scale DAC current. The relationship between R
IOUT
= 18 × FSADJUST/R
FULL SCALE
FSADJUST = 1.14 V nominal, R
19 REFOUT Voltage Reference Output. The AD9838 has an internal 1.20 V reference that is available at this pin.
20 COMP DAC Bias Pin. This pin is used for decoupling the DAC bias voltage.
EP Exposed Pad. Connect the exposed pad to ground.
) is connected between this pin and AGND to determine the magnitude
SET
SET
= 6.8 kΩ typical
SET
and the full-scale current is as follows:
SET
Rev. A | Page 8 of 32
AD9838
–
–
–
TYPICAL PERFORMANCE CHARACTERISTICS
6.0
5.5
5.0
(mA)
4.5
AA
+ I
DD
I
4.0
3.5
VDD = 5V
VDD = 3V
40
AVDD = DVDD = 3V
T
–45
0Hz TO NYQUIST
–50
–55
–60
SFDR (dB)
–65
–70
–75
= 25°C
A
MCLK/7
MCLK/50
3.0
024681012141618
Figure 6. Typical Current Consumption (I
2.5
2.0
1.5
(mA)
DD
I
1.0
0.5
0
024681012141618
Figure 7. Typical Current Consumption (I
91
AVDD = DVDD = 3V
= 25°C
T
A
–92
±200kHz
–93
–94
–95
SFDR (dB)
–96
–97
–98
MCLK FREQUENCY (MHz)
+ IAA) vs. MCLK Frequency
DD
= MCLK/10
for f
OUT
VDD = 5V
VDD = 3V
MCLK FREQUENCY ( MHz)
for f
OUT
MCLK/50
= MCLK/10
DD
MCLK/7
) vs. MCLK Frequency
–80
1611
09077-020
MCLK FREQUENCY (MHz)
16
09077-023
Figure 9. Wideband SFDR vs. MCLK Frequency
40
–45
–50
–55
SNR (dB)
–60
–65
–70
024681012141618
09077-021
MCLK FREQUENCY (MHz)
09077-024
Figure 10. SNR vs. MCLK Frequency
1000
900
800
700
600
WAKE-UP TIME (µs)
500
VDD = 5.5V
VDD = 2.3V
–99
02 46810121416
MCLK FREQUENCY (MHz)
Figure 8. Narrow-Band SFDR vs. MCLK Frequency
09077-022
400
–40 –20 0 20 40 60 80 100 120 140
TEMPERATURE ( °C)
Figure 11. Wake-Up Time vs. Temperature
09077-037
Rev. A | Page 9 of 32
AD9838
1.180
1.178
1.176
1.174
(V)
1.172
REF
V
1.170
1.168
1.166
1.164
–40 –20 0 20 40 60 80 100 120 140
VDD = 2.7V
TEMPERATURE (°C)
Figure 12. V
VDD = 5.0V
vs. Temperature
REF
09077-038
0
–10
–20
–30
–40
–50
–60
POWER (dB)
–70
–80
–90
–100
0 102030405060708090100
Figure 15. Power vs. Frequency, f
FREQUENCY (kHz)
= 16 MHz, f
MCLK
= 3.8 kHz,
OUT
Frequency Word = 0x000FBA9
09077-047
0.20
0.18
0.16
DVDD = 2.3V
0.14
0.12
0.10
DVDD (V)
0.08
0.06
0.04
0.02
0
–40 –20 0 20 40 60 80 100
Figure 13. SIGN BIT OUT Pin, Low Level, I
5.5
5.0
4.5
4.0
3.5
DVDD (V)
3.0
2.5
2.0
1.5
–40 –20 0 20 40 60 80 100
Figure 14. SIGN BIT OUT Pin, High Level, I
DVDD = 3.3V
TEMPERATURE (° C)
DVDD = 5.5V
DVDD = 4.5V
DVDD = 3.3V
DVDD = 2.7V
DVDD = 2.3V
TEMPERATURE (° C)
DVDD = 5.5V
= 1 mA
SINK
= 1 mA
SINK
0
–10
–20
–30
–40
–50
–60
POWER (dB)
–70
–80
–90
–100
09077-045
0 10 2030405060708090100
Figure 16. Power vs. Frequency, f
FREQUENCY (kHz)
= 5 MHz, f
MCLK
= 1.2 kHz,
OUT
09077-048
Frequency Word = 0x000FBA9
0
–10
–20
–30
–40
–50
–60
POWER (dB)
–70
–80
–90
–100
09077-046
0 0.51.01.52.02.
Figure 17. Power vs. Frequency, f
FREQUENCY (MHz)
= 5 MHz, f
MCLK
= 0.714 MHz = f
OUT
5
09077-049
/7,
MCLK
Frequency Word = 0x2492492
Rev. A | Page 10 of 32
AD9838
0
–10
–20
–30
–40
–50
POWER (dB)
–60
–70
–80
–90
02461357
Figure 18. Power vs. Frequency, f
FREQUENCY (MHz)
= 16 MHz, f
MCLK
Frequency Word = 0x2492492
= 2.28 MHz,
OUT
8
09077-050
Rev. A | Page 11 of 32
AD9838
F
TEST CIRCUIT
R
SET
100n
10nF
6.8kΩ
COMP
IOUT
AVDD
R
LOAD
200Ω
10nF
20pF
09077-002
CAP/2.5V
REGULATOR
AD9838
ON-BOARD
REFERENCE
12
SIN
ROM
REFOUT
FSADJUST
FULL-SCALE
CONTROL
10-BIT DAC
Figure 19. Test Circuit Used to Test Specifications
Rev. A | Page 12 of 32
AD9838
TERMINOLOGY
Integral Nonlinearity (INL) Total Harmonic Distortion (THD)
INL is the maximum deviation of any code from a straight line
passing through the endpoints of the transfer function. The
endpoints of the transfer function are zero scale, a point 0.5 LSB
below the first code transition (000 … 00 to 000 … 01), and full
scale, a point 0.5 LSB above the last code transition (111 … 10
to 111 … 11). The error is expressed in LSBs.
Differential Nonlinearity (DNL)
DNL is the difference between the measured and ideal 1 LSB
change between two adjacent codes in the DAC. A specified
DNL of ±1 LSB maximum ensures monotonicity.
Output Compliance
Output compliance refers to the maximum voltage that can be
generated at the output of the DAC to meet the specifications.
When voltages greater than that specified for the output compliance are generated, the AD9838 may not meet the specifications
listed in the data sheet.
Spurious-Free Dynamic Range (SFDR)
Along with the frequency of interest, harmonics of the fundamental frequency and images of these frequencies are present at
the output of a DDS device. The spurious-free dynamic range
(SFDR) refers to the largest spur or harmonic present in the
band of interest. The wideband SFDR gives the magnitude of
the largest spur or harmonic relative to the magnitude of the
fundamental frequency in the 0 to Nyquist bandwidth. The
narrow-band SFDR gives the attenuation of the largest spur or
harmonic in a bandwidth of ±200 kHz about the fundamental
frequency.
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the rms value of the fundamental. For the AD9838,
THD is defined as
2
THD
=
2
log20
4
3
V
1
VVVVV
++++
6
5
2
2
2
2
where:
V
is the rms amplitude of the fundamental.
1
V
, V3, V4, V5, and V6 are the rms amplitudes of the second
2
through sixth harmonics.
Signal-to-Noise Ratio (SNR)
SNR is the ratio of the rms value of the measured output signal
to the rms sum of all other spectral components below the
Nyquist frequency. The value for SNR is expressed in decibels.
Clock Feedthrough
There is feedthrough from the MCLK input to the analog
output. Clock feedthrough refers to the magnitude of the
MCLK signal relative to the fundamental frequency in the
output spectrum of the AD9838.
Rev. A | Page 13 of 32
AD9838
THEORY OF OPERATION
Sine waves are typically thought of in terms of their magnitude
form: a(t) = sin(ωt). However, sine waves are nonlinear and not
easy to generate except through piecewise construction. On the
other hand, the angular information is linear in nature; that is,
the phase angle rotates through a fixed angle for each unit of
time. The angular rate depends on the frequency of the signal
MAGNITUDE
2π
PHASE
2π
f.
4π
4π
6π
6π
09077-025
by the traditional rate of ω = 2π
+1
0
–1
28
2
0
Figure 20. Sine Wave
Knowing that the phase of a sine wave is linear and given a
reference interval (clock period), the phase rotation for that
period can be determined as follows:
ΔPhase = ωΔt (1)
Solving for ω,
ω = ΔPhase/Δt = 2πf (2)
Solving for f and substituting the reference clock frequency for
the reference period (1/f
f = ΔPhase × f
MCLK
= Δt),
MCLK
∕2π (3)
The AD9838 builds the output based on this simple equation. A
simple DDS chip can implement this equation with three major
subcircuits: numerically controlled oscillator (NCO) plus phase
modulator, SIN ROM, and digital-to-analog converter (DAC).
Each subcircuit is described in the Circuit Description section.
Rev. A | Page 14 of 32
AD9838
CIRCUIT DESCRIPTION
The AD9838 is a fully integrated direct digital synthesis (DDS)
chip. The chip requires one reference clock, one low precision
resistor, and eight decoupling capacitors to provide digitally
created sine waves up to 8 MHz. In addition to the generation
of this RF signal, the chip is fully capable of a broad range of
simple and complex modulation schemes. These modulation
schemes are fully implemented in the digital domain, allowing
accurate and simple realization of complex modulation algorithms using DSP techniques.
The internal circuitry of the AD9838 consists of the following
main sections: a numerically controlled oscillator (NCO),
frequency and phase modulators, SIN ROM, a digital-to-analog
converter, a comparator, and a regulator.
NUMERICALLY CONTROLLED OSCILLATOR PLUS PHASE MODULATOR
The AD9838 consists of two frequency select registers, a phase
accumulator, two phase offset registers, and a phase offset adder.
The main component of the NCO is a 28-bit phase accumulator.
Continuous time signals have a phase range of 0 to 2π. Outside
this range of numbers, the sinusoid functions repeat themselves
in a periodic manner. The digital implementation is no different.
The accumulator simply scales the range of phase numbers into
a multibit digital word. The phase accumulator in the AD9838
is implemented with 28 bits. Therefore, in the AD9838, 2π = 2
28
Likewise, the ΔPhase term is scaled into this range of numbers:
ΔPhase < 2
0 <
28
− 1
With these substitutions, Equation 3 becomes
f = ΔPhase × f
where 0 < Δ
Phase < 2
∕228 (4)
MCLK
28
− 1.
The input to the phase accumulator can be selected from either
the FREQ0 register or the FREQ1 register and is controlled by
the FSELECT pin or the FSEL bit in the control register. NCOs
inherently generate continuous phase signals, thus avoiding any
output discontinuity when switching between frequencies.
Following the NCO, a phase offset can be added to perform
phase modulation using the 12-bit phase registers. The contents
of one of these phase registers is added to the MSBs of the NCO.
The AD9838 has two phase registers; their resolution is 2π/4096.
SIN ROM
To make the output from the NCO useful, it must be converted
from phase information into a sinusoidal value. Because phase
information maps directly to amplitude, the SIN ROM uses the
digital phase information as an address to a lookup table and
converts the phase information into amplitude.
.
Although the NCO contains a 28-bit phase accumulator, the
output of the NCO is truncated to 12 bits. Using the full resolution of the phase accumulator is impractical and unnecessary
28
because a lookup table of 2
entries would be required. It is only
necessary to have sufficient phase resolution such that the errors
due to truncation are smaller than the resolution of the 10-bit
DAC. Therefore, the SIN ROM must have two bits of phase
resolution more than the 10-bit DAC.
The SIN ROM is enabled using the OPBITEN and MODE bits
(Bit D5 and Bit D1) in the control register (see Tab l e 1 9 ).
DIGITAL-TO-ANALOG CONVERTER (DAC)
The AD9838 includes a high impedance, current source, 10-bit
DAC capable of driving a wide range of loads. The full-scale
output current can be adjusted for optimum power and external
load requirements using a single external resistor (R
SET
).
The DAC can be configured for single-ended or differential
operation. The IOUT and IOUTB pins can be connected through
equal external resistors to AGND to develop complementary
output voltages. The load resistors can be of any value required,
as long as the full-scale voltage developed across them does not
exceed the output compliance range. Because full-scale current
is controlled by R
, adjustments to R
SET
can balance changes
SET
made to the load resistors.
COMPARATOR
The AD9838 can be used to generate synthesized digital clock
signals. This is accomplished by using the on-board self-biasing
comparator that converts the sinusoidal signal of the DAC to a
square wave. The output from the DAC can be filtered externally
before being applied to the comparator input. The comparator
reference voltage is the time average of the signal applied to VIN.
The comparator can accept signals in the range of approximately
100 mV p-p to 1 V p-p. The comparator input is ac-coupled;
therefore, to operate correctly as a zero-crossing detector, the
comparator requires a minimum input frequency of 3 MHz typical. The comparator output is a square wave with an amplitude
from 0 V to DVDD.
The AD9838 provides a sampled signal with its output following Nyquist sampling theorem. Specifically, its output spectrum
contains the fundamental plus aliased signals (images) that occur
at multiples of the reference clock frequency and the selected
output frequency. A graphical representation of the sampled
spectrum, with aliased images, is shown in Figure 21.
Rev. A | Page 15 of 32
AD9838
The prominence of the aliased images depends on the ratio of
to MCLK. If the ratio is small, the aliased images are very
f
OUT
prominent and of a relatively high energy level as determined by
the sin(x)/x roll-off of the quantized DAC output. In fact, depending on the f
/reference clock ratio, the first aliased image can
OUT
be on the order of −3 dB below the fundamental.
A low-pass filter is generally placed between the output of the
DAC and the input of the comparator to further suppress the
effects of aliased images. To avoid unwanted (and unexpected)
output anomalies, it is necessary to consider the relationship of
the selected output frequency and the reference clock frequency.
To apply the AD9838 as a clock generator, limit the selected
output frequency to <33% of the reference clock frequency. In
this way, the user can prevent the generation of aliased signals
that fall within, or close to, the output band of interest (generally
the dc selected output frequency). This practice reduces the
complexity (and cost) of the external filter requirement for the
clock generator application. For more information, see the
AN-837 Application Note.
To enable the comparator, the SIGN/PIB and OPBITEN bits in
the control resister must be set to 1 (see Tab l e 1 8 ).
REGULATOR
The AD9838 has separate power supplies for the analog and
digital sections. AVDD provides the power supply required for
the analog section, and DVDD provides the power supply for
the digital section. Both supplies can have a value of 2.3 V to
5.5 V and are independent of each other. For example, the
analog section can be operated at 5 V, and the digital section
can be operated at 3 V, or vice versa.
The internal digital section of the AD9838 is operated at 2.5 V.
An on-board regulator steps down the voltage applied at DVDD
to 2.5 V. The digital interface (serial port) of the AD9838 also
operates from DVDD. These digital signals are level shifted
within the AD9838 to make them 2.5 V compatible.
If the voltage applied at the DVDD pin of the AD9838 is less than
or equal to 2.7 V, the CAP/2.5V and DVDD pins should be tied
together to bypass the on-board regulator.
f
OUT
SIGNAL AMPLITUDE
0Hz FIRST
sin(x)/x ENVELOPE
f/f
x = π (
f
–
f
C
OUT
IMAGE
SYSTEM CLOCK
)
C
f
C
f
+
f
C
OUT
SECOND
IMAGE
2
f
–
f
C
OUT
THIRD
IMAGE
FREQUENCY (Hz)
2
f
C
2
f
+
C
FOURTH
IMAGE
f
OUT
3
f
C
IMAGE
–
f
FIFTH
OUT
3
f
+
f
C
3
OUT
f
C
SIXTH
IMAGE
09077-040
Figure 21. DAC Output Spectrum
Rev. A | Page 16 of 32
AD9838
FUNCTIONAL DESCRIPTION
and t
SERIAL INTERFACE
The AD9838 has a standard 3-wire serial interface that is
compatible with the SPI, QSPI™, MICROWIRE®, and DSP
interface standards.
Data is loaded into the device as a 16-bit word under the control
of a serial clock input, SCLK. The timing diagram for this operation is given in Figure 4.
FSYNC is a level triggered input that acts as a frame synchronization and chip enable input. Data can be transferred into the
device only when FSYNC is low. To start the serial data transfer,
FSYNC should be taken low, observing the minimum FSYNC
to SCLK falling edge setup time, t
(see Tabl e 2 ). After FSYNC
7
goes low, serial data is shifted into the input shift register of the
device on the falling edges of SCLK for 16 clock pulses. FSYNC
can be taken high after the 16th falling edge of SCLK, observing
the minimum SCLK falling edge to FSYNC rising edge time, t
8
Alternatively, FSYNC can be kept low for a multiple of 16 SCLK
pulses and then brought high at the end of the data transfer. In
this way, a continuous stream of 16-bit words can be loaded
while FSYNC is held low; FSYNC goes high only after the 16th
SCLK falling edge of the last word loaded.
The SCLK can be continuous, or it can idle high or low between
write operations. In either case, it must be high when FSYNC
goes low (t
).
12
For an example of how to program the AD9838, see the AN-1070
Application Note on the Analog Devices, Inc., website. The
AD9838 has the same register settings as the AD9833/AD9834.
LATENCY PERIOD
A latency period is associated with each operation. When the
FSELECT and PSELECT pins change value, there is a pipeline
delay before control is transferred to the selected register.
SLEEP12
SLEEP1
RESET
MODE + OPBI TEN
SIGN/PIB
OPBITEN
ACCUMULATOR
PHASE
(28-BIT)
SIN
ROM
MUX
0
MUX
1
DIVIDE
0
BY 2
1
When the t
11
FSELECT and PSELECT have latencies of eight MCLK cycles.
When the t
and t
11
is increased by one MCLK cycle.
Similarly, a latency period is associated with each asynchronous
write operation. If a selected frequency or phase register is
loaded with a new word, there is a delay of eight or nine MCLK
cycles before the analog output changes. The delay can be eight
or nine MCLK cycles, depending on the position of the MCLK
rising edge when the data is loaded into the destination register.
The negative transitions of the RESET and SLEEP pins are
sampled on the internal falling edge of MCLK. Therefore, they
also have a latency period associated with them.
CONTROL REGISTER
.
The AD9838 contains a 16-bit control register that allows the
user to configure the operation of the AD9838. All control bits
other than the MODE bit are sampled on the internal falling
edge of MCLK.
Figure 22 illustrates the functions of the control bits. Tabl e 7
describes the individual bits of the control register. The different
functions and the various output options of the AD9838 are
described in more detail in the following sections.
To inform the AD9838 that the contents of the control register
will be altered, Bit D15 and Bit D14 must be set to 0, as shown
in Tab l e 6 .
Table 6. Control Register Bits
D15 D14 D13 to D0
0 0 Control bits
(LOW POWER)
MSB
COMPARATOR
1
MUX
0
timing specifications are met (see Figure 3),
11A
timing specifications are not met, the latency
11A
IOUT
10-BIT DAC
DIGITAL
OUTPUT
(ENABLE)
IOUTB
VIN
SIGN BIT OUT
D150D140D13
B28
D12
HLB
D11
D10
FSEL
PSELD9PIN/SWD8RESETD7SLEEP1D6SLEEP12D5OPBITEND4SIGN/PIBD3DIV2D20D1MODED00
Figure 22. Function of Control Bits
Rev. A | Page 17 of 32
09077-026
AD9838
Table 7. Control Register Bit Descriptions
Bit Bit Name Description
D13 B28 Two write operations are required to load a complete word into either of the frequency registers.
B28 = 1 allows a complete word to be loaded into a frequency register in two consecutive writes. The first write
contains the 14 LSBs of the frequency word, and the second write contains the 14 MSBs. The first two bits of each
16-bit word define the frequency register to which the word is loaded and should, therefore, be the same for both
consecutive writes. See Table 11 for the appropriate addresses. The write to the frequency register occurs after both
words have been loaded, so the register never holds an intermediate value. An example of a complete 28-bit write
is shown in Tab le 12. Note, however, that consecutive 28-bit writes to the same frequency register are not allowed;
to execute consecutive 28-bit writes, you must alternate between the frequency registers.
B28 = 0 configures the 28-bit frequency register to operate as two 14-bit registers, one containing the 14 MSBs and
the other containing the 14 LSBs. In this way, the 14 MSBs of the frequency word can be altered independently of
the 14 LSBs, and vice versa. To alter the 14 MSBs or the 14 LSBs, a single write is made to the appropriate frequency
address. Bit D12 (HLB) informs the AD9838 whether the bits to be altered are the 14 MSBs or the 14 LSBs.
D12 HLB
D11 FSEL The FSEL bit defines whether the FREQ0 register or the FREQ1 register is used in the phase accumulator (see Table 9).
D10 PSEL
D9 PIN/SW
D8 RESET When the PIN/SW bit is set to 0, this bit controls the reset function.
D7 SLEEP1 This bit enables or disables the internal MCLK.
D6 SLEEP12 When the PIN/SW bit is set to 0, this bit powers down the on-chip DAC.
D5 OPBITEN
D4 SIGN/PIB This bit controls the output at the SIGN BIT OUT pin when the OPBITEN bit (Bit D5) is set to 1.
D3 DIV2 DIV2 is used when the OPBITEN bit (Bit D5) is set to 1 and the SIGN/PIB bit (Bit D4) is set to 0 (see Table 18 ).
D2 Reserved This bit must be set to 0.
D1 MODE
D0 Reserved This bit must be set to 0.
This control bit allows the user to continuously load the MSBs or LSBs of a frequency register while ignoring the
remaining 14 bits. This is useful if the complete 28-bit resolution is not required. The HLB bit is used in conjunction
with the B28 bit (Bit D13). The HLB bit indicates whether the 14 bits to be loaded are transferred to the 14 MSBs or
the 14 LSBs of the addressed frequency register. Bit D13 (B28) must be set to 0 to change the MSBs or LSBs of a
frequency word separately. When Bit D13 (B28) is set to 1, the HLB bit is ignored.
HLB = 1 allows a write to the 14 MSBs of the addressed frequency register.
HLB = 0 allows a write to the 14 LSBs of the addressed frequency register.
The PSEL bit defines whether the PHASE0 register data or the PHASE1 register data is added to the output of the
phase accumulator (see Table 10 ).
The following functions can be implemented using either software or hardware: frequency register selection, phase
register selection, reset of internal registers, and DAC power-down. The PIN/SW bit selects the source of control for
these functions.
PIN/SW = 1 selects the control pins to implement the register selection, reset, and DAC power-down functions.
PIN/SW = 0 selects the control bits to implement the register selection, reset, and DAC power-down functions.
RESET = 1 resets internal registers to 0, which corresponds to an analog output of midscale.
RESET = 0 disables the reset function (see the Reset Function section).
SLEEP1 = 1 disables the internal MCLK. The DAC output remains at its present value because the NCO is no longer
accumulating.
SLEEP1 = 0 enables the internal MCLK (see the Sleep Function section).
SLEEP12 = 1 powers down the on-chip DAC. This is useful when the AD9838 is used to output the MSB of the DAC data.
SLEEP12 = 0 implies that the DAC is active (see the Sleep Function section).
This bit controls whether an output is available at the SIGN BIT OUT pin. If the user is not using the SIGN BIT OUT
pin, this bit should be set to 0.
OPBITEN = 1 enables the SIGN BIT OUT pin.
OPBITEN = 0 places the SIGN BIT OUT output buffer into a high impedance state (no output is available at the SIGN
BIT OUT pin).
SIGN/PIB = 1 connects the on-board comparator to the SIGN BIT OUT pin. After filtering the sinusoidal output from
the DAC, the waveform can be applied to the comparator to generate a square waveform (see Table 18).
SIGN/PIB = 0 connects the MSB (or MSB/2) of the DAC data to the SIGN BIT OUT pin. Bit D3 (DIV2) controls whether
the output is the MSB or MSB/2.
DIV2 = 1 causes the MSB of the DAC data to be output at the SIGN BIT OUT pin.
DIV2 = 0 causes the MSB/2 of the DAC data to be output at the SIGN BIT OUT pin.
This bit, in association with the OPBITEN bit (Bit D5), controls the output at the IOUT and IOUTB pins. This bit should
be set to 0 if the OPBITEN bit is set to 1 (see Table 19).
MODE = 1 bypasses the SIN ROM, resulting in a triangle output from the DAC.
MODE = 0 uses the SIN ROM to convert the phase information into amplitude information, resulting in a sinusoidal
signal at the output.
Rev. A | Page 18 of 32
AD9838
FREQUENCY AND PHASE REGISTERS
The AD9838 contains two frequency registers and two phase
registers, which are described in Ta ble 8.
Table 8. Frequency and Phase Registers
Register Size Description
FREQ0 28 bits
FREQ1 28 bits
PHASE0 12 bits
PHASE1 12 bits
The analog output from the AD9838 is
f
/228 × FREQREG
MCLK
where FREQREG is the value loaded into the selected frequency
register.
This signal is phase shifted by
2π/4096 ×
PHASEREG is the value contained in the selected phase
where
register.
The relationship of the selected output frequency and the reference clock frequency must be considered to avoid unwanted
output anomalies.
Selecting a Frequency or Phase Register
Access to the frequency and phase registers is controlled by
the FSELECT and PSELECT pins or by the FSEL and PSEL
control bits. If the PIN/SW control bit (Bit D9) = 1, the pins
control the function; if the PIN/SW control bit = 0, the bits
control the function (see Tab l e 9 and Ta ble 10 ). If the FSEL
and PSEL bits are used, the pins should be held at CMOS logic
high or low. Control of the frequency and phase registers is
interchangeable from the pins to the bits.
Table 9. Selecting a Frequency Register
FSELECT Pin FSEL Bit PIN/SW Bit Selected Register
0 X 1 FREQ0
1 X 1 FREQ1
X 0 0 FREQ0
X 1 0 FREQ1
Frequency Register 0.
When the FSEL bit or FSELECT pin = 0, the
FREQ0 register defines the output frequency
as a fraction of the MCLK frequency.
Frequency Register 1.
When the FSEL bit or FSELECT pin = 1, the
FREQ1 register defines the output frequency
as a fraction of the MCLK frequency.
Phase Offset Register 0.
When the PSEL bit or PSELECT pin = 0, the
contents of the PHASE0 register are added
to the output of the phase accumulator.
Phase Offset Register 1.
When the PSEL bit or PSELECT pin = 1, the
contents of the PHASE1 register are added
to the output of the phase accumulator.
PHASEREG
Table 10. Selecting a Phase Register
PSELECT Pin PSEL Bit PIN/SW Bit Selected Register
0 X 1 PHASE0
1 X 1 PHASE1
X 0 0 PHASE0
X 1 0 PHASE1
The FSELECT and PSELECT pins are sampled on the internal
falling edge of MCLK. It is recommended that the data on these
pins not change within the time window of the falling edge of
MCLK (see Figure 3 for timing). If the FSELECT or PSELECT
pin changes value when a falling edge occurs, there is an uncertainty of one MCLK cycle as it pertains to when control is
transferred to the other frequency/phase register.
The flowcharts in Figure 26 and Figure 27 show the routine for
selecting and writing to the frequency and phase registers of the
AD9838.
Writing to a Frequency Register
When writing to a frequency register, Bit D15 and Bit D14 of
the control register give the address of the frequency register
(see Tabl e 1 1 ).
Table 11. Frequency Register Bits
D15 D14 D13 to D0
0 1 14 FREQ0 register bits
1 0 14 FREQ1 register bits
To change the entire contents of a frequency register, two consecutive writes to the same address must be performed because the
frequency registers are 28 bits wide. The first write contains the
14 LSBs, and the second write contains the 14 MSBs. For this
mode of operation, the B28 control bit (Bit D13) must be set
to 1. An example of a 28-bit write is shown in Tabl e 12 .
Table 12. Writing 0xFFFC000 to the FREQ0 Register
SDATA Input Result of Input Word
0010 0000 0000 0000
0100 0000 0000 0000
0111 1111 1111 1111
Control word write
(D15, D14 = 00), B28 (D13) = 1,
HLB (D12) = X
FREQ0 register write
(D15, D14 = 01), 14 LSBs = 0x0000
FREQ0 register write
(D15, D14 = 01), 14 MSBs = 0x3FFF
Note, however, that continuous writes to the same frequency
register may result in intermediate updates during the writes. If
a frequency sweep, or something similar, is required, it is recommended that users alternate between the two frequency registers.
Rev. A | Page 19 of 32
AD9838
In some applications, the user does not need to alter all 28 bits
of the frequency register. With coarse tuning, only the 14 MSBs
are altered; with fine tuning, only the 14 LSBs are altered. By
setting the B28 control bit (Bit D13) to 0, the 28-bit frequency
register operates as two 14-bit registers, one containing the
14 MSBs and the other containing the 14 LSBs. In this way, the
14 MSBs of the frequency word can be altered independently
of the 14 LSBs, and vice versa. The HLB bit (Bit D12) in the
control register identifies which 14 bits are being altered (see
Tabl e 13 and Tabl e 1 4 ).
Table 13. Writing 0x3FFF to the 14 LSBs of the FREQ1 Register
SDATA Input Result of Input Word
0000 0000 0000 0000
1011 1111 1111 1111
Control word write
(D15, D14 = 00), B28 (D13) = 0,
HLB (D12) = 0, that is, LSBs
FREQ1 register write
(D15, D14 = 10), 14 LSBs = 0x3FFF
Table 16. Applying the Reset Function
RESET Pin RESET Bit PIN/SW Bit Result
0 X 1 No reset applied
1 X 1 Internal registers reset
X 0 0 No reset applied
X 1 0 Internal registers reset
The effect of asserting the RESET pin is immediately evident
at the output—that is, the 0-to-1 transition of this pin is not
sampled. However, the negative (1-to-0) transition of the
RESET pin is sampled on the internal falling edge of MCLK.
SLEEP FUNCTION
Sections of the AD9838 that are not in use can be powered
down to minimize power consumption by using the sleep
function. The parts of the chip that can be powered down are
the internal clock and the DAC. The DAC can be powered
down using hardware or software (see Tab l e 17 ).
Table 14. Writing 0x00FF to the 14 MSBs of the FREQ0 Register
SDATA Input Result of Input Word
0001 0000 0000 0000
0100 0000 1111 1111
Control word write
(D15, D14 = 00), B28 (D13) = 0,
HLB (D12) = 1, that is, MSBs
FREQ0 register write
(D15, D14 = 01), 14 MSBs = 0x00FF
Writing to a Phase Register
When writing to a phase register, Bit D15 and Bit D14 are set
to 11. Bit D13 identifies the phase register that is being loaded.
Table 15. Phase Register Bits
D15 D14 D13 D12 D11 to D0
1 1 0 X 12 PHASE0 register bits
1 1 1 X 12 PHASE1 register bits
RESET FUNCTION
The reset function resets the appropriate internal registers to 0
to provide an analog output of midscale. A reset does not reset
the phase, frequency, or control registers. When the AD9838 is
powered up, the part should be reset (see the Powering Up the
AD9838 section). To reset the AD9838, set the RESET pin or bit
to 1. To take the part out of reset, set the RESET pin or bit to 0.
A signal appears at the DAC output eight or nine MCLK cycles
after the RESET pin or bit is set to 0.
The reset function is controlled by either the RESET pin or the
RESET control bit. If the PIN/SW control bit = 0, the RESET bit
controls the function; if the PIN/SW control bit = 1, the RESET
pin controls the function (see Tabl e 16 ).
Table 17. Applying the Sleep Function
SLEEP
Pin
0 X X 1 No power-down
1 X X 1 DAC powered down
X 0 0 0 No power-down
X 0 1 0 DAC powered down
X 1 0 0 Internal clock disabled
X 1 1 0
SLEEP1
Bit
SLEEP12
Bit
PIN/SW
Bit Result
DAC powered down and
internal clock disabled
DAC Powered Down
When the AD9838 is used to output the MSB of the DAC data
only, the DAC is not required. The DAC can be powered down
to reduce power consumption.
Internal Clock Disabled
When the internal clock of the AD9838 is disabled, the DAC
output remains at its present value because the NCO is no longer
accumulating. New frequency, phase, and control words can be
written to the part when the SLEEP1 control bit is active. Because
the synchronizing clock (FSYNC) remains active, the selected
frequency and phase registers can also be changed either at the
pins or by using the control bits. Setting the SLEEP1 bit to 0
enables the MCLK. Any changes made to the registers while
SLEEP1 was active are observed at the output after a latency
period (see the Latency Period section).
The effect of asserting the SLEEP pin is immediately evident
at the output—that is, the 0-to-1 transition of this pin is not
sampled. However, the negative (1-to-0) transition of the SLEEP
pin is sampled on the internal falling edge of MCLK.
Rev. A | Page 20 of 32
AD9838
V
SIGN BIT OUT PIN
The AD9838 offers a variety of outputs from the chip. The
digital outputs are available from the SIGN BIT OUT pin. The
available outputs are the comparator output or the MSB of the
DAC data. The bits controlling the SIGN BIT OUT pin are
listed in Tab le 1 8.
Table 18. Outputs from the SIGN BIT OUT Pin
OPBITEN
Bit
0 X X X High impedance
1 0 0 0 DAC data MSB/2
1 0 0 1 DAC data MSB
1 0 1 0 Reserved
1 0 1 1 Comparator output
1 1 X X Reserved
The SIGN BIT OUT pin must be enabled before use. The
OPBITEN bit (Bit D5) in the control register enables and disables this pin. When OPBITEN = 1, the SIGN BIT OUT pin is
enabled. Note that if OPBITEN = 1, the MODE bit (Bit D1) in
the control register should be set to 0.
Comparator Output
The AD9838 has an on-board comparator. To connect this comparator to the SIGN BIT OUT pin, the OPBITEN bit (Bit D5)
and the SIGN/PIB bit (Bit D4) must be set to 1. After filtering
the sinusoidal output from the DAC, the waveform can be
applied to the comparator to generate a square waveform.
MSB of the DAC Data
The MSB of the DAC data can be output from the AD9838.
By setting the OPBITEN bit (Bit D5) to 1 and the SIGN/PIB bit
(Bit D4) to 0, the MSB of the DAC data is available at the SIGN
BIT OUT pin. This output is useful as a coarse clock source.
The square wave can also be divided by 2 before being output.
The DIV2 bit (Bit D3) in the control register controls the
frequency of this output from the SIGN BIT OUT pin.
MODE
Bit
SIGN/PIB
Bit
DIV2
Bit SIGN BIT OUT Pin
IOUT AND IOUTB PINS
The analog outputs from the AD9838 are available from the
IOUT and IOUTB pins. The available outputs are a sinusoidal
output or a triangular output (see Ta b le 1 9 ).
Note that the SLEEP pin and the SLEEP12 bit must be set to 0
(the DAC is enabled) when using the IOUT and IOUTB pins.
Table 19. Outputs from the IOUT and IOUTB Pins
OPBITEN Bit MODE Bit IOUT and IOUTB Pin Output
0 0 Sinusoid
0 1 Triangle
1 0 Sinusoid
1 1 Reserved
Sinusoidal Output
The SIN ROM converts the phase information from the
frequency and phase registers into amplitude information,
resulting in a sinusoidal signal at the output. To obtain a
sinusoidal output from the IOUT and IOUTB pins, set the
MODE bit (Bit D1) to 0.
Triangle Output
The SIN ROM can be bypassed so that the truncated digital output
from the NCO is sent to the DAC. In this case, the output is no
longer sinusoidal. The DAC produces a 10-bit linear triangular
function (see Figure 23). To obtain a triangle output from the
IOUT and IOUTB pins, set the MODE bit (Bit D1) to 1 and the
OPBITEN bit (Bit D5) to 0.
OUT MAX
V
OUT MIN
3π/2 7π/2 11π/2
Figure 23. Triangle Output
POWERING UP THE AD9838
The flowchart in Figure 24 shows the operating routine for the
AD9838. When the AD9838 is powered up, the part should be
reset. This resets the appropriate internal registers to 0 to provide
an analog output of midscale. To avoid spurious DAC outputs
during AD9838 initialization, the RESET pin or the RESET bit
should be set to 1 until the part is ready to begin generating
an output.
A reset does not reset the phase, frequency, or control registers.
These registers will contain invalid data and, therefore, should
be set to known values by the user. The RESET pin or bit should
then be set to 0 to begin generating an output. The data appears
on the DAC output eight or nine MCLK cycles after the RESET
pin or bit is set to 0.
09077-027
Rev. A | Page 21 of 32
AD9838
DATA WRITE
(SEE FIGURE 26)
SELECT DATA
SOURCES
(SEE FIGURE 27)
= V
V
OUT
YES
CHANGE FSEL /
FSELECT?
YES
CHANGE FREQUE NCY
REGISTER?
CONTROL
REGISTER
(SEE TABLE 7)
REFOUT
WRITE
NO
× 18 × R
(SEE TIMING DIAGRAM
× (1 + (SIN(2π(FREQREG ×
LOAD/RSET
YES
CHANGE FREQUENCY?
CHANGE DAC OUTPUT
FROM SIN TO TRIANGLE?
YES
YES
CHANGE OUTPUT AT
SIGN BIT OUT PIN?
WAIT 8/9 MCLK
CYCLES
FIGURE 3)
DAC OUTPUT
CHANGE PHASE?
NO
NO
NO
NO
f
MCLK
YES
INITIALIZATION
(SEE FIGURE 25)
×t/228 + PHASEREG/212))))
CHANGE PSEL/
PSELECT?
NO
CHANGE PHASE
REGISTER?
YES
YES
09077-028
Figure 24. Flowchart for AD9838 Initialization and Operation
INITIALIZATION
USING CONTROL
BIT
(CONTROL REGISTER WRITE)
RESET = 1
PIN/SW = 0
WRITE TO FREQUENCY AND PHASE REGISTERS
FREQ0 REG = f
PHASE0 AND PHASE1 REG = (PHASESHIFT × 212)/2π
USING CONTROL
BITS
(CONTROL REGISTER WRITE)
FSEL = SELECTED FREQUENCY REGISTER
PSEL = SELECTED PHASE REGISTER
RESET BIT = 0
PIN/SW = 0
FREQ1 REG = f
SELECT FREQUENCY REGISTERS
SELECT PHASE REGISTERS
Figure 25. Flowchart for Initialization
APPLY RESET
OUT0/fMCLK
OUT1/fMCLK
(SEE FIGURE 26)
SET RESET = 0
FSELECT = SELECTED FREQUENCY REGISTER
Rev. A | Page 22 of 32
USING PI N
(CONTROL REGISTER WRITE)
PIN/SW = 1
28
× 2
28
× 2
USING PI NS
(APPLY SIGNALS AT PINS)
RESET PIN = 0
PSELECT = SELECTED PHASE REGISTER
PIN/SW = 1
SET RESET PIN = 1
09077-029
AD9838
DATA WRITE
WRITE A F ULL 28-BIT WORD
TO A FREQUENCY REGIST ER?
(CONTROL REGISTER WRITE)
B28 (D13) = 1
WRITE T WO CONSECUT IVE
16-BIT WO RDS
(SEE TABLE 12 FOR EXAMPLE)
WRITE ANOTHER FULL
28-BIT WORD TO A
FREQUENCY REG ISTER?
YES
YES
NO
WRITE 14 M SBs OR LSBs
TO A FREQUENCY REGIST ER?
YES
(CONTROL REGISTER WRITE)
B28 (D13) = 0
HLB (D12) = 0/1
WRITE A 16-BIT WORD
(SEE TABLE 13 AND TABLE 14
FOR EXAMPL ES)
WRITE 14 MSBs OR LSBs
FREQUENCY REGISTER?
TO A
NO
NONO
D15, D14 = 11
D11 ... D0 = PHASE DATA
YES
WRITE TO PHASE
REGISTER?
YES
(16-BIT WRITE)
D13 = 0/1 (CHOO SE THE
D12 = X
PHASE REGISTER)
WRITE TO ANOTHER
PHASE REGISTER?
NO
YES
09077-030
Figure 26. Flowchart for Data Writes
SELECT DATA S OURCES
FSELECT AND PSELECT
PINS BEING USED?
YES
SET FSELECT AND
PSELECT PINS
NO
(CONTROL REGISTER WRITE)
PIN/SW = 0
SET FSEL BIT
SET PSEL BIT
(CONTROL REGISTER WRITE)
PIN/SW = 1
09077-031
Figure 27. Flowchart for Selecting Data Sources
Rev. A | Page 23 of 32
AD9838
APPLICATIONS INFORMATION
The various output options available from the AD9838 make
the part suitable for a wide variety of applications, including
modulation applications. The AD9838 can be used to perform
simple modulation, such as frequency shift keying (FSK). More
complex modulation schemes, such as Gaussian minimum shift
keying (GMSK) and quadrature phase shift keying (QPSK), can
also be implemented using the AD9838.
In an FSK application, the two frequency registers of the AD9838
are loaded with different values. One frequency represents the
space frequency, and the other represents the mark frequency. The
digital data stream is fed to the FSELECT pin, causing the AD9838
to modulate the carrier frequency between the two values.
The AD9838 has two phase registers, enabling the part to perform phase shift keying (PSK). With PSK, the carrier frequency
is phase shifted, that is, the phase is altered by an amount that
is related to the bit stream input to the modulator.
The AD9838 is also suitable for signal generator applications.
Using the on-board comparator, the device can be used to generate a square wave.
With its low current consumption, the part is also suitable for
applications in which it can be used as a local oscillator.
GROUNDING AND LAYOUT
The printed circuit board that houses the AD9838 should be
designed so that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the use
of ground planes that can be separated easily. A minimum etch
technique is generally best for ground planes because it provides
the best shielding. Digital and analog ground planes should be
joined in one place only. If the AD9838 is the only device that
requires an AGND to DGND connection, the ground planes
should be connected at the AGND and DGND pins of the
AD9838. If the AD9838 is in a system where multiple devices
require AGND to DGND connections, the connection should
be made at one point only, a star ground point that should be
established as close as possible to the AD9838.
Avoid running digital lines under the device; these lines couple
noise onto the die. The analog ground plane should be allowed
to run under the AD9838 to avoid noise coupling. The power
supply lines to the AD9838 should use as large a track as possible
to provide low impedance paths and reduce the effects of glitches
on the power supply line. Fast switching signals, such as clocks,
should be shielded with digital ground to avoid radiating noise
to other sections of the board.
Rev. A | Page 24 of 32
Avoid crossover of digital and analog signals. Traces on opposite
sides of the board should run at right angles to each other to
reduce the effects of feedthrough through the board. A microstrip technique is by far the best but is not always possible with
a double-sided board. In this technique, the component side of
the board is dedicated to ground planes and signals are placed
on the other side.
Good decoupling is important. The analog and digital supplies
to the AD9838 are independent and separately pinned out to
minimize coupling between the analog and digital sections of the
device. All analog and digital supplies should be decoupled to
AGND and DGND, respectively, with 0.1 μF ceramic capacitors
in parallel with 10 μF tantalum capacitors. To achieve the best
performance from the decoupling capacitors, they should be
placed as close as possible to the device, ideally right up against
the device.
In systems where a common supply is used to drive both the
AVDD and DVDD pins of the AD9838, it is recommended that
the system’s AVDD supply be used. This supply should have the
recommended analog supply decoupling between the AVDD
pin of the AD9838 and AGND, as well as the recommended
digital supply decoupling capacitors between the DVDD pin
and DGND.
Proper operation of the comparator requires good layout strategy.
The layout must minimize the parasitic capacitance between V
and the SIGN BIT OUT pin by using a ground plane to add
isolation. For example, in a multilayered board, the V
can be connected to the top layer, and the SIGN BIT OUT pin
can be connected to the bottom layer. In this way, isolation is
provided by the power and ground planes between V
SIGN BIT OUT pin.
IN
signal
IN
and the
IN
INTERFACING TO MICROPROCESSORS
The AD9838 has a standard serial interface that allows the part to
interface directly with several microprocessors. The device uses
an external serial clock to write the data or control information
into the device. The serial clock can have a frequency of 40 MHz
maximum. The serial clock can be continuous, or it can idle high
or low between write operations. When data or control information is written to the AD9838, FSYNC is taken low and is held
low until the 16 bits of data are written into the AD9838. The
FSYNC signal frames the 16 bits of information that are loaded
into the AD9838.
AD9838
AD9838 to 68HC11/68L11 Interface
Figure 28 shows the serial interface between the AD9838 and
the 68HC11/68L11 microcontroller. The microcontroller is configured as the master by setting the MSTR bit in the SPCR to 1.
This setting provides a serial clock on SCK; the MOSI output
drives the serial data line, SDATA. Because the microcontroller
does not have a dedicated frame sync pin, the FSYNC signal is
derived from a port line (PC7). The setup conditions for correct
operation of the interface are as follows:
• SCK idles high between write operations (CPOL = 0)
• Data is valid on the SCK falling edge (CPHA = 1)
When data is to be transmitted to the AD9838, the FSYNC line
(PC7) is taken low. Serial data from the 68HC11/68L11 is transmitted in 8-bit bytes with only eight falling clock edges occurring
in the transmit cycle. Data is transmitted MSB first. To load data
into the AD9838, PC7 is held low after the first eight bits are
transferred, and a second serial write operation is performed to
the AD9838. Only after the second eight bits are transferred
should FSYNC be taken high again.
68HC11/68L11
PC7
MOSI
SCK
Figure 28. 68HC11/68L11 to AD9838 Interface
AD9838
FSYNC
SDATA
SCLK
09077-033
AD9838 to 80C51/80L51 Interface
Figure 29 shows the serial interface between the AD9838 and
the 80C51/80L51 microcontroller. The microcontroller is operated in Mode 0 so that TxD of the 80C51/80L51 drives SCLK of
the AD9838, and RxD drives the serial data line, SDATA. The
FSYNC signal is derived from a bit programmable pin on the
port (P3.3 is shown in Figure 29).
When data is to be transmitted to the AD9838, P3.3 is taken low.
The 80C51/80L51 transmits data in 8-bit bytes with only eight
falling SCLK edges occurring in each cycle. To load the remaining eight bits to the AD9838, P3.3 is held low after the first eight
bits are transmitted, and a second write operation is initiated to
transmit the second byte of data. P3.3 is taken high following
the completion of the second write operation. SCLK should idle
high between the two write operations.
The 80C51/80L51 outputs the serial data in a format that has the
LSB first. The AD9838 accepts the MSB first (the four MSBs are
the control information, the next four bits are the address, and
the eight LSBs contain the data when writing to a destination
register). Therefore, the transmit routine of the 80C51/80L51
must take this into account and rearrange the bits so that the
MSB is output first.
80C51/80L51
P3.3
RXD
TXD
Figure 29. 80C51/80L51 to AD9838 Interface
AD9838
FSYNC
SDATA
SCLK
9077-034
AD9838 to DSP56002 Interface
Figure 30 shows the interface between the AD9838 and the
DSP56002. The DSP56002 is configured for normal mode asynchronous operation with a gated internal clock (SYN = 0, GCK = 1,
SCKD = 1). The frame sync pin is generated internally (SC2 = 1),
the transfers are 16 bits wide (WL1 = 1, WL0 = 0), and the frame
sync signal frames the 16 bits (FSL = 0). The frame sync signal is
available on the SC2 pin, but it must be inverted before it is applied
to the AD9838. The interface to the DSP56000/DSP56001 is
similar to that of the DSP56002.
DSP56002
SC2
STD
SCK
Figure 30. DSP56002 to AD9838 Interface
AD9838
FSYNC
SDATA
SCLK
09077-035
Rev. A | Page 25 of 32
AD9838
EVALUATION BOARD
The AD9838 evaluation board allows designers to evaluate the
high performance AD9838 DDS modulator with a minimum
of effort.
SYSTEM DEMONSTRATION PLATFORM
The system demonstration platform (SDP) is a hardware and
software evaluation tool for use in conjunction with product
evaluation boards. The SDP board is based on the Blackfin®
ADSP-BF527 processor with USB connectivity to the PC
through a USB 2.0 high speed port. For more information,
see the SDP board product page.
Note that the SDP board is sold separately from the AD9838
evaluation board.
AD9838 TO SPORT INTERFACE
The Analog Devices SDP board has a SPORT serial port that is
used to control the serial inputs to the AD9838. The connections
are shown in Figure 31.
ADSP-BF527
SPORT_TFS
SPORT_TSCLK
SPORT_DT0
Figure 31. SDP to AD9838 Interface
AD9838
FSYNC
SCLK
SDATA
09077-041
EVALUATION KIT
The DDS evaluation kit includes a populated, tested AD9838
printed circuit board (PCB). The schematics of the evaluation
board are shown in Figure 33 and Figure 34.
The software provided in the evaluation kit allows the user to
easily program the AD9838 (see Figure 32). The evaluation software runs on any IBM-compatible PC with Microsoft® Windows®
software installed (including Windows 7). The software is compatible with both 32-bit and 64-bit operating systems.
More information about the evaluation software is available on
the software CD and on the AD9838 product page.
Figure 32. AD9838 Evaluation Software Interface
CRYSTAL OSCILLATOR VS. EXTERNAL CLOCK
The AD9838 can operate with master clocks up to 16 MHz.
A 16 MHz oscillator is included on the evaluation board. This
oscillator can be removed and, if required, an external CMOS
clock can be connected to the part. Options for the general
oscillator include the following:
• AEL 301-Series oscillators, AEL Crystals
• SG-310SCN oscillators, Epson Electronics
POWER SUPPLY
Power to the AD9838 evaluation board can be provided from
the USB connector or externally through pin connections. The
power leads should be twisted to reduce ground loops.
09077-036
Rev. A | Page 26 of 32
AD9838
EVALUATION BOARD SCHEMATICS
09077-042
Figure 33. Evaluation Board Schematic
Rev. A | Page 27 of 32
AD9838
09077-043
Figure 34. SDP Connector Schematic
Rev. A | Page 28 of 32
AD9838
EVALUATION BOARD LAYOUT
09077-044
Figure 35. Evaluation Board Layout
Rev. A | Page 29 of 32
AD9838
OUTLINE DIMENSIONS
PIN 1
INDICATOR
0.80
0.75
0.70
SEATING
PLANE
4.10
4.00 SQ
3.90
0.50
BSC
0.50
0.40
0.30
0.05 MAX
0.02 NOM
0.20 REF
COPLANARITY
0.08
0.30
0.25
0.20
16
15
EXPOSED
11
10
BOTTOM VIEWTOP VIEW
20
1
PAD
5
6
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
P
N
I
N
I
D
I
0.25 MIN
1
A
O
T
C
2.65
2.50 SQ
2.35
R
COMPLIANTTOJEDEC STANDARDS MO-220-WGGD.
061609-B
Figure 36. 20-Lead Lead Frame Chip Scale Package [LFCSP_WQ]
4 mm × 4 mm Body, Very Very Thin Quad
(CP-20-10)
Dimensions shown in millimeters
ORDERING GUIDE
1, 2
Model
AD9838BCPZ-RL −40°C to +125°C 16 MHz 20-Lead Lead Frame Chip Scale Package [LFCSP_WQ] CP-20-10
AD9838BCPZ-RL7 −40°C to +125°C 16 MHz 20-Lead Lead Frame Chip Scale Package [LFCSP_WQ] CP-20-10
AD9838ACPZ-RL −40°C to +125°C 5 MHz 20-Lead Lead Frame Chip Scale Package [LFCSP_WQ] CP-20-10
AD9838ACPZ-RL7 −40°C to +125°C 5 MHz 20-Lead Lead Frame Chip Scale Package [LFCSP_WQ] CP-20-10
EVAL-AD9838SDZ Evaluation Board
1
Z = RoHS Compliant Part.
2
The evaluation board for the AD9838 requires the system demonstration platform (SDP) board, which is sold separately.
Temperature Range Max MCLK Package Description Package Option
Rev. A | Page 30 of 32
AD9838
NOTES
Rev. A | Page 31 of 32
AD9838
NOTES
©2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D09077-0-4/11(A)
Rev. A | Page 32 of 32
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