Datasheet AD9835 Datasheet (Analog Devices)

REV. 0
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a
AD9835
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 1998
50 MHz CMOS
Complete DDS
FUNCTIONAL BLOCK DIAGRAM
IOUT
COMP
REFINFS ADJUST
REFOUT
AGND
AVDDDGND
DVDD
MCLK
PSEL0
PSEL1
12
Σ
AD9835
ON-BOARD
REFERENCE
10-BIT
DAC
PHASE0 REG PHASE1 REG PHASE2 REG PHASE3 REG
FULL-SCALE
CONTROL
COS ROM
PHASE
ACCUMULATOR
(32 BIT)
MUX
FREQ0 REG
FREQ1 REG
MUX
16-BIT DATA REGISTER
SYNC
FSELECT
FSELECT
BIT
SELSRC
SYNC
8 LSBs8 MSBs
DECODE LOGIC
FSYNC SCLK SDATA
SERIAL REGISTER
CONTROL REGISTER
FSELECT/PSEL REGISTER
DEFER REGISTER
SYNC
SYNC
SELSRC
PSEL0
BIT
PSEL1
BIT
FEATURES 5 V Power Supply 50 MHz Speed On-Chip COS Look-Up Table On-Chip 10-Bit DAC Serial Loading Power-Down Option 200 mW Power Consumption 16-Lead TSSOP
APPLICATIONS DDS Tuning Digital Demodulation
GENERAL DESCRIPTION
The AD9835 is a numerically controlled oscillator employing a phase accumulator, a COS Look-Up Table and a 10-bit D/A converter integrated on a single CMOS chip. Modulation capabilities are provided for phase modulation and frequency modulation.
Clock rates up to 50 MHz are supported. Frequency accuracy can be controlled to one part in 4 billion. Modulation is ef­fected by loading registers through the serial interface. A power-down bit allows the user to power down the AD9835 when it is not in use, the power consumption being reduced to 1.75 mW. The part is available in a 16-lead TSSOP package.
–2– REV. 0
AD9835–SPECIFICATIONS
1
(VDD = +5 V 5%; AGND = DGND = 0 V; TA = T
MIN
to T
MAX
; REFIN = REFOUT;
R
SET
= 3.9 k; R
LOAD
= 300 for IOUT, unless otherwise noted)
Parameter AD9835B Units Test Conditions/Comments
SIGNAL DAC SPECIFICATIONS
Resolution 10 Bits Update Rate (f
MAX
) 50 MSPS nom
IOUT Full Scale 4 mA
nom
4.75 mA max Output Compliance 1.35 V max DC Accuracy
Integral Nonlinearity ±1 LSB typ Differential Nonlinearity ±0.5 LSB typ
DDS SPECIFICATIONS
2
Dynamic Specifications
Signal-to-Noise Ratio 50 dB min f
MCLK
= 50 MHz, f
OUT
= 1 MHz
Total Harmonic Distortion –52 dBc max f
MCLK
= 50 MHz, f
OUT
= 1 MHz
Spurious Free Dynamic Range (SFDR)
3
f
MCLK
= 6.25 MHz, f
OUT
= 2.11 MHz
Narrow Band (±50 kHz) –72 dBc min Wide Band (±2 MHz) –50 dBc min
Clock Feedthrough –60 dBc typ Wake-Up Time 1 ms typ Power-Down Option Yes
VOLTAGE REFERENCE
Internal Reference @ +25°C 1.21 V typ
T
MIN
to T
MAX
1.21 ± 7% V min/max REFIN Input Impedance 10 M typ Reference TC 100 ppm/°C typ REFOUT Output Impedance 300 typ
LOGIC INPUTS
V
INH
, Input High Voltage DVDD – 0.9 V min
V
INL
, Input Low Voltage 0.9 V max
I
INH
, Input Current 10 µA
max
CIN, Input Capacitance 10 pF max
POWER SUPPLIES f
MCLK
= 50 MHz
AVDD 4.75/5.25 V
min/V max
DVDD 4.75/5.25 V
min/V max
I
AA
5mA
max
I
DD
2.5 + 0.33/MHz mA typ I
AA
+ I
DD
4
40 mA max
Low Power Sleep Mode 0.35 mA max
NOTES
1
Operating temperature range is as follows: B Version: –40 °C to +85°C.
2
100% production tested.
3
f
MCLK
= 6.25 MHz, Frequency Word = 5671C71C HEX, f
OUT
= 2.11 MHz.
4
Measured with the digital inputs static and equal to 0 V or DVDD. The AD9835 is tested with a capacitive load of 50 pF. The part can be operated with higher capacitive loads, but the magnitude of the analog output will be attenuated. See Figure 5.
Specifications subject to change without notice.
IOUT
COMP
REFIN
FS ADJUST
REFOUT
12
AD9835
ON-BOARD
REFERENCE
10-BIT
DAC
COS ROM
FULL-SCALE
CONTROL
300V 50pF
R
SET
3.9kV
10nF
10nF
AVDD
Figure 1. Test Circuit with Which Specifications Are Tested
AD9835
–3–REV. 0
TIMING CHARACTERISTICS
(VDD = +5 V 5%; AGND = DGND = 0 V, unless otherwise noted)
Limit at T
MIN
to T
MAX
Parameter (B Version) Units Test Conditions/Comments
t
1
20 ns min MCLK Period
t
2
8 ns min MCLK High Duration
t
3
8 ns min MCLK Low Duration
t
4
50 ns min SCLK Period
t
5
20 ns min SCLK High Duration
t
6
20 ns min SCLK Low Duration
t
7
15 ns min FSYNC to SCLK Falling Edge Setup Time
t
8
20 ns min FSYNC to SCLK Hold Time SCLK – 5 ns max
t
9
15 ns min Data Setup Time
t
10
5 ns min Data Hold Time
t
11
8 ns min FSELECT, PSEL0, PSEL1 Setup Time Before MCLK Rising Edge
t
11A
1
8 ns min FSELECT, PSEL0, PSEL1 Setup Time After MCLK Rising Edge
NOTES
1
See Pin Description section.
Guaranteed by design but not production tested.
MCLK
t
2
t
1
t
3
Figure 2. Master Clock
SCLK
FSYNC
SDATA
t
5
t
4
t
6
t
7
t
8
t
10
t
9
D15 D14 D2 D1 D0 D15 D14
Figure 3. Serial Timing
t
11A
t
11
VALID DATA VALID DATA VALID DATA
MCLK
FSELECT
PSEL0, PSEL1
Figure 4. Control Timing
AD9835
–4– REV. 0
ABSOLUTE MAXIMUM RATINGS*
(T
A
= +25°C unless otherwise noted)
AVDD to AGND . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
DVDD to DGND . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
AVDD to DVDD . . . . . . . . . . . . . . . . . . . . . –0.3 V to +0.3 V
AGND to DGND. . . . . . . . . . . . . . . . . . . . . .–0.3 V to +0.3 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 +85°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Maximum Junction Temperature . . . . . . . . . . . . . . . .+150°C
TSSOP θ
JA
Thermal Impedance . . . . . . . . . . . . . . . 158°C/W
Lead Temperature, Soldering
Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . .+215°C
Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . .+220°C
ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . > 4500 V
*Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating condi­tions for extended periods may affect device reliability.
ORDERING GUIDE
Temperature Package Package
Model Range Description Option*
AD9835BRU –40°C to +85°C 16-Lead TSSOP RU-16
*RU = Thin Shrink Small Outline Package (TSSOP).
PIN CONFIGURATION
14 13 12 11
16 15
10
9
8
1 2 3 4
7
6
5
TOP VIEW
(Not to Scale)
AD9835
FS ADJUST
AGND
IOUT
AVDD
COMP
REFIN
REFOUT
DVDD
FSELECT
PSEL1
PSEL0
DGND MCLK
SCLK
SDATA
FSYNC
TERMINOLOGY Integral Nonlinearity
This 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
This is the difference between the measured and ideal 1 LSB change between two adjacent codes in the DAC.
Signal to (Noise + Distortion)
Signal to (Noise + Distortion) is measured signal to noise at the output of the DAC. The signal is the rms magnitude of the fundamental. Noise is the rms sum of all the nonfundamental signals up to half the sampling frequency (f
MCLK
/2) but exclud­ing the dc component. Signal to (Noise + Distortion) is depen­dent on the number of quantization levels used in the digitization process; the more levels, the smaller the quantization noise. The theoretical Signal to (Noise + Distortion) ratio for a sine wave input is given by
Signal to (Noise + Distortion) = (6.02N + 1.76) dB
where N is the number of bits. Thus, for an ideal 10-bit con­verter, Signal to (Noise + Distortion) = 61.96 dB.
Total Harmonic Distortion
Total Harmonic Distortion (THD) is the ratio of the rms sum of harmonics to the rms value of the fundamental. For the AD9835, THD is defined as
THD = 20 log
(V
2
2
+V
3
2
+V
4
2
+V
5
2
+V
6
2
)
V
1
where V1 is the rms amplitude of the fundamental and V2, V3, V
4
, V5 and V6 are the rms amplitudes of the second through the
sixth harmonic.
Output Compliance
The 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 AD9835 may not meet the speci­fications listed in the data sheet.
Spurious Free Dynamic Range
Along with the frequency of interest, harmonics of the funda­mental frequency and images of the MCLK frequency 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 harmonic or spur relative to the magni-
tude of the fundamental frequency in the bandwidth ±2 MHz
about the fundamental frequency. The narrow band SFDR gives the attenuation of the largest spur or harmonic in a bandwidth of
±50 kHz about the fundamental frequency.
Clock Feedthrough
There will be 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 AD9835’s output spectrum.
AD9835
–5–REV. 0
PIN FUNCTION DESCRIPTIONS
Pin # Mnemonic Function
ANALOG SIGNAL AND REFERENCE
1 FS ADJUST Full-Scale Adjust Control. A resistor (R
SET
) is connected between this pin and AGND. This determines
the magnitude of the full-scale DAC current. The relationship between R
SET
and the full-scale current is
as follows:
IOUT
FULL-SCALE
= 12.5 × V
REFIN/RSET
V
REFIN
= 1.21 V nominal, R
SET
= 3.9 ktypical
2 REFIN Voltage Reference Input. The AD9835 can be used with either the onboard reference, which is available
from pin REFOUT, or an external reference. The reference to be used is connected to the REFIN pin. The AD9835 accepts a reference of 1.21 V nominal.
3 REFOUT Voltage Reference Output. The AD9835 has an onboard reference of value 1.21 V nominal. The refer-
ence is made available on the REFOUT pin. This reference is used as the reference to the DAC by con­necting REFOUT to REFIN. REFOUT should be decoupled with a 10 nF capacitor to AGND.
14 IOUT Current Output. This is a high impedance current source. A load resistor should be connected between
IOUT and AGND.
16 COMP Compensation pin. This is a compensation pin for the internal reference amplifier. A 10 nF decoupling
ceramic capacitor should be connected between COMP and AVDD.
POWER SUPPLY
4 DVDD Positive Power Supply for the Digital Section. A 0.1 µF decoupling capacitor should be connected be-
tween DVDD and DGND. DVDD can have a value of +5 V ± 5%.
5 DGND Digital Ground. 13 AGND Analog Ground.
15 AVDD Positive Power Supply for the Analog Section. A 0.1 µF decoupling capacitor should be connected be-
tween AVDD and AGND. AVDD can have a value of +5 V ± 5%.
DIGITAL INTERFACE AND CONTROL
6 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.
7 SCLK Serial Clock, Logic Input. Data is clocked into the AD9835 on each falling SCLK edge. 8 SDATA Serial Data In, Logic Input. The 16-bit serial data word is applied to this input. 9 FSYNC Data Synchronization Signal, Logic Input. When this input is taken low, the internal logic is informed
that a new word is being loaded into the device.
10 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 pin FSELECT or the bit FSELECT. FSELECT is sampled on the rising MCLK edge. FSELECT needs to be in steady state when an MCLK rising edge occurs. If FSELECT changes value when a rising edge occurs, there is an uncertainty of one MCLK cycle as to when control is transferred to the other frequency register. To avoid any uncertainty, a change on FSELECT should not coincide with an MCLK rising edge. When the bit is being used to select the frequency register, the pin FSELECT should be tied to DGND.
11, 12 PSEL0, PSEL1 Phase Select Input. The AD9835 has four phase registers. These registers can be used to alter the value
being input to the COS ROM. The contents of the phase register are added to the phase accumula­tor output, the inputs PSEL0 and PSEL1 selecting the phase register to be used. Alternatively, the phase register to be used can be selected using bits PSEL0 and PSEL1. Like the FSELECT input, PSEL0 and PSEL1 are sampled on the rising MCLK edge. Therefore, these inputs need to be in steady state when an MCLK rising edge occurs or there is an uncertainty of one MCLK cycle as to when control is transferred to the selected phase register. When the phase registers are being con­trolled by the bits PSEL0 and PSEL1, the pins should be tied to DGND.
AD9835
–6– REV. 0
Table V. Commands
C3 C2 C1 C0 Command
0 0 0 0 Write 16 Phase bits (Present 8 Bits + 8 Bits
in Defer Register) to Selected PHASE
REG. 0 0 0 1 Write 8 Phase bits to Defer Register. 0 0 1 0 Write 16 Frequency bits (Present 8 Bits
+ 8 Bits in Defer Register) to Selected
FREQ REG. 0 0 1 1 Write 8 Frequency bits to Defer Register. 0 1 0 0 Bits D9 (PSEL0) and D10 (PSEL1) are
used to Select the PHASE REG when
SELSRC = 1. When SELSRC = 0, the
PHASE REG is selected using the pins
PSEL0 and PSEL1 Respectively. 0 1 0 1 Bit D11 is used to select the FREQ REG
when SELSRC = 1. When SELSRC = 0,
the FREQ REG is selected using the pin
FSELECT. 0 1 1 0 This command is used to control the
PSEL0, PSEL1 and FSELECT bits
using only one write. Bits D9 and D10
are used to select the PHASE REG and
Bit 11 is used to select the FREQ REG
when SELSRC = 1. When SELSRC = 0,
the PHASE REG is selected using the
pins PSEL0 and PSEL1 and the FREQ
REG is selected using the pin FSELECT. 0 1 1 1 Reserved. Configures the AD9835 for
Test Purposes.
Table VI. Controlling the AD9835
D15 D14 Command
1 0 Selects source of Control for the PHASE and
FREQ Registers and Enables Synchronization. Bit D13 is the SYNC Bit. When this bit is High, read­ing of the FSELECT, PSEL0 and PSEL1 bits/pins and the loading of the Destination Register with data is synchronized with the rising edge of MCLK. The latency is increased by 2 MCLK cycles when SYNC = 1. When SYNC = 0, the loading of the data and the sampling of FSELECT/PSEL0/PSEL1 occurs asynchronously. Bit D12 is the Select Source Bit (SELSRC). When this bit Equals 1, the PHASE/FREQ REG is Selected using the bits FSELECT, PSEL0 and PSEL1. When SELSRC = 0, the PHASE/FREQ REG is Selected using the pins FSELECT, PSEL0 and PSEL1.
1 1 Sleep, Reset and Clear. D13 is the SLEEP bit. When
this bit equals 1, the AD9835 is powered down, inter­nal clocks are disabled and the DAC's current sources and REFOUT are turned off. When SLEEP = 0, the AD9835 is powered up. When RESET (D12) = 1, the phase accumulator is set to zero phase which corresponds to an analog output of full scale. When CLR (D11) = 1, SYNC and SELSRC are set to zero. CLR automatically resets to zero.
Table I. Control Registers
Register Size Description
FREQ0 REG 32 Bits Frequency Register 0. This de-
fines the output frequency, when FSELECT = 0, as a fraction of the MCLK frequency.
FREQ1 REG 32 Bits Frequency Register 1. This de-
fines the output frequency, when FSELECT = 1, as a fraction of the MCLK frequency.
PHASE0 REG 12 Bits Phase Offset Register 0. When
PSEL0 = PSEL1 = 0, the contents of this register are added to the output of the phase accumulator.
PHASE1 REG 12 Bits Phase Offset Register 1. When
PSEL0 = 1 and PSEL1 = 0, the con­tents of this register are added to the output of the phase accumulator.
PHASE2 REG 12 Bits Phase Offset Register 2. When
PSEL0 = 0 and PSEL1 = 1, the contents of this register are added to the output of the phase accumulator.
PHASE3 REG 12 Bits Phase Offset Register 3. When
PSEL0 = PSEL1 = 1, the contents of this register are added to the output of the phase accumulator.
Table II. Addressing the Registers
A3 A2 A1 A0 Destination Register
0 0 0 0 FREG0 REG 8 L LSBs 0 0 0 1 FREG0 REG 8 H LSBs 0 0 1 0 FREG0 REG 8 L MSBs 0 0 1 1 FREG0 REG 8 H MSBs 0 1 0 0 FREG1 REG 8 L LSBs 0 1 0 1 FREG1 REG 8 H LSBs 0 1 1 0 FREG1 REG 8 L MSBs 0 1 1 1 FREG1 REG 8 H MSBs 1 0 0 0 PHASE0 REG 8 LSBs 1 0 0 1 PHASE0 REG 8 MSBs 1 0 1 0 PHASE1 REG 8 LSBs 1 0 1 1 PHASE1 REG 8 MSBs 1 1 0 0 PHASE2 REG 8 LSBs 1 1 0 1 PHASE2 REG 8 MSBs 1 1 1 0 PHASE3 REG 8 LSBs 1 1 1 1 PHASE3 REG 8 MSBs
Table III. 32-Bit Frequency Word
sBSM61sBSL61
sBSMH8sBSML8sBSLH8sBSLL8
Table IV. 12-Bit Frequency Word
ehtfosBSM4ehT(sBSM4
)0=dedaoLdroWtiB-8
sBSL8
AD9835
–7–REV. 0
Table VII. Writing to the AD9835 Data Registers
51D41D31D21D11D01D9D8D7D6D5D4D3D2D1D0D
3C2C1C0C3A2A1A0ABSMBSL
Table VIII. Setting SYNC and SELSRC
51D41D31D21D11D01D9D8D7D6D5D4D3D2D1D0D
10 CNYSCRSLES XXXXXXXXXXXX
Table IX. Power-Down, Resetting and Clearing the AD9835
51D41D31D21D11D01D9D8D7D6D5D4D3D2D1D0D
11 PEELSTESERRLC XXXXXXXXXXX
Typical Performance Characteristics
OUTPUT FREQUENCY – MHz
0
–12
0162
SIGNAL ATTENUATION – dB
468101214
–2
–4
–6
–8
–10
CL = 82pF
CL = 100pF
CL = 150pF
AVDD = DVDD = +5V
Figure 5. Signal Attenuation vs. Output Frequency for Various Capacitive Load (R
L
= 300 Ω)
MCLK FREQUENCY – MHz
30
25
20
15
10
0
10 5020
TOTAL CURRENT – mA
30 40
5
TA = +258C AVDD = DVDD = +5V
Fi gure 6. Typical Current Consumption vs. MCLK Frequency
MCLK FREQUENCY – MHz
–64
–66
–68
–70
–72
–76
–74
10 5020
SFDR (650kHz) – dB
30 40
f
OUT/fMCLK
= 1/3
AVDD = DVDD = +5V
Figure 7. Narrow Band SFDR vs. MCLK Frequency
MCLK FREQUENCY – MHz
0
–10
–20
–30
–40
–60
–50
10 5020
SFDR (62MHz) – dB
30 40
f
OUT/fMCLK
= 1/3
AVDD = DVDD = +5V
Figure 8. Wide Band SFDR vs. MCLK Frequency
AD9835
–8– REV. 0
f
OUT/fMCLK
–20
–30
–40
–50
–60
–80
–70
0.044 0.3640.124
0.204 0.284
SFDR (62MHz) – dB
0.084 0.164
0.244 0.324
AVDD = DVDD = +5V
50MHz
30MHz
10MHz
Figure 9. Wide Band SFDR vs. f
OUT/fMCLK
for Various MCLK
Frequencies
MCLK FREQUENCY – MHz
56
55
54
53
52
50
10 5020
SNR – dB
30 40
51
f
OUT/fMCLK
= 1/3
AVDD = DVDD = +5V
Figure 10. SNR vs. MCLK Frequency
f
OUT/fMCLK
70
60
50
40
30
10
20
0.044 0.3640.124
0.204 0.284
SNR – dB
0.084 0.164
0.244 0.324
AVDD = DVDD = +5V
50MHz
30MHz
10MHz
Figure 11. SNR vs. f
OUT/fMCLK
for Various MCLK
Frequencies
RBW 1kHz VBW 3kHz ST 50 SEC
10dB/DIV
0Hz
START
25MHz
STOP
Figure 12. f
MCLK
= 50 MHz, f
OUT
= 2.1 MHz. Frequency
Word = ACO8312
RBW 1kHz VBW 3kHz ST 50 SEC
10dB/DIV
0Hz
START
25MHz
STOP
Figure 13. f
MCLK
= 50 MHz, f
OUT
= 3.1 MHz. Frequency
Word = FDF3B64
RBW 1kHz VBW 3kHz ST 50 SEC
10dB/DIV
0Hz
START
25MHz
STOP
Figure 14. f
MCLK
= 50 MHz, f
OUT
= 7.1 MHz. Frequency
Word = 245AICAC
AD9835
–9–REV. 0
RBW 1kHz VBW 3kHz ST 50 SEC
10dB/DIV
0Hz
START
25MHz
STOP
Figure 15. f
MCLK
= 50 MHz, f
OUT
= 9.1 MHz. Frequency
Word = 2E978D50
RBW 1kHz VBW 3kHz ST 50 SEC
10dB/DIV
0Hz
START
25MHz
STOP
Figure 16. f
MCLK
= 50 MHz, f
OUT
= 11.1 MHz. Frequency
Word = 38D4FDF4
RBW 1kHz VBW 3kHz ST 50 SEC
10dB/DIV
0Hz
START
25MHz
STOP
Figure 17. f
MCLK
= 50 MHz, f
OUT
= 13.1 MHz. Frequency
Word = 43126E98
RBW 1kHz VBW 3kHz ST 50 SEC
10dB/DIV
0Hz
START
25MHz
STOP
Figure 18. f
MCLK
= 50 MHz, f
OUT
= 16.5 MHz. Frequency
Word = 547AE148
AD9835
–10– REV. 0
CIRCUIT DESCRIPTION
The AD9835 provides an exciting new level of integration for the RF/Communications system designer. The AD9835 com­bines the Numerical Controlled Oscillator (NCO), COS Look-Up Table, Frequency and Phase Modulators, and a Digital-to­Analog Converter on a single integrated circuit.
The internal circuitry of the AD9835 consists of three main sections. These are:
Numerical Controlled Oscillator (NCO) + Phase Modulator COS Look-Up Table Digital-to-Analog Converter
The AD9835 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 25 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.
THEORY OF OPERATION
Cos waves are typically thought of in terms of their magnitude
form a(t) = cos (ωt). However, these are nonlinear and not easy
to generate except through piece-wise 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
by the traditional rate of ω = 2 πf.
MAGNITUDE
PHASE
+1
0
–1
2π
0
Figure 19. Cos Wave
Knowing that the phase of a cos wave is linear and given a refer­ence interval (clock period), the phase rotation for that period can be determined.
Phase = ωδt
Solving for ω
ω = Phase/δt = 2 πf
Solving for f and substituting the reference clock frequency for the reference period (1/f
MCLK
= δt)
f = Phase × f
MCLK
/2 π
The AD9835 builds the output based on this simple equation. A simple DDS chip can implement this equation with three major subcircuits.
Numerical Controlled Oscillator and Phase Modulator
This consists of two frequency select registers, a phase accumu­lator and four phase offset registers. The main component of the NCO is a 32-bit phase accumulator which assembles the phase
component of the output signal. Continuous time signals have a
phase range of 0 π to 2 π. 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 AD9835 is implemented with
32 bits. Therefore, in the AD9835, 2 π = 2
32
. Likewise, the
Phase term is scaled into this range of numbers 0 < Phase
< 2
32
– 1. Making these substitutions into the equation above
f = Phase × f
MCLK
/2
32
where 0 < Phase < 2
32
The input to the phase accumulator (i.e., the phase step) can be selected either from the FREQ0 Register or FREQ1 Register and this is controlled by the FSELECT pin or the FSELECT bit. 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 this register are added to the most significant bits of the NCO. The AD9835 has four PHASE registers, the resolution
of these registers being 2 π/4096.
COS Look-Up Table (LUT)
To make the output useful, the signal must be converted from phase information into a sinusoidal value. Since phase informa­tion maps directly into amplitude, a ROM LUT converts the phase information into amplitude. To do this, the digital phase information is used to address a COS ROM LUT. Although the NCO contains a 32-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 as this would require a look-up table of 2
32
entries.
It is necessary only to have sufficient phase resolution in the LUTs such that the dc error of the output waveform is domi­nated by the quantization error in the DAC. This requires the look-up table to have two more bits of phase resolution than the 10-bit DAC.
Digital-to-Analog Converter
The AD9835 includes a high impedance current source 10-bit DAC, capable of driving a wide range of loads at different speeds. Full-scale output current can be adjusted, for optimum power and external load requirements, through the use of a single external resistor (R
SET
).
The DAC is configured for single-ended operation. The load resistor can be any value required, as long as the full-scale volt­age developed across it does not exceed the voltage compliance range. Since full-scale current is controlled by R
SET
, adjust-
ments to R
SET
can balance changes made to the load resistor. However, if the DAC full-scale output current is significantly less than 4 mA, the DAC’s linearity may degrade.
DSP and MPU Interfacing
The AD9835 has a serial interface, with 16 bits being loaded during each write cycle. SCLK, SDATA and FSYNC are used to load the word into the AD9835. When FSYNC is taken low, the AD9835 is informed that a word is being written to the device. The first bit is read into the device on the next SCLK falling edge with the remaining bits being read into the device on the subsequent SCLK falling edges. FSYNC frames the 16 bits; therefore, when 16 SCLK falling edges have occurred,
AD9835
–11–REV. 0
FSYNC should be taken high again. The SCLK can be con­tinuous or, alternatively, the SCLK can idle high or low be­tween write operations.
When writing to a frequency/phase register, the first four bits identify whether a frequency or phase register is being written to, the next four bits contain the address of the destination register while the 8 LSBs contain the data. Table II lists the addresses for the phase/frequency registers while Table III lists the commands.
Within the AD9835, 16-bit transfers are used when loading the destination frequency/phase register. There are two modes for loading a register—direct data transfer and a deferred data transfer. With a deferred data transfer, the 8-bit word is loaded into the defer register (8 LSBs or 8 MSBs). However, this data is not loaded into the 16-bit data register so the destination register is not updated. With a direct data transfer, the 8-bit word is loaded into the appropriate defer register (8 LSBs or 8 MSBs). Immediately following the loading of the defer regis­ter, the contents of the complete defer register are loaded into the 16-bit data register and the destination register is loaded on the next MCLK rising edge. When a destination register is addressed, a deferred transfer is needed first, followed by a direct transfer. When all 16 bits of the defer register contain relevant data, the destination register can then be updated using 8-bit loading rather than 16-bit loading, i.e., direct data trans­fers can be used. For example, after a new 16-bit word has been loaded to a destination register, the defer register will also contain this word. If the next write instruction is to the same destination register, the user can use direct data transfers immediately.
When writing to a phase register, the 4 MSBs of the 16-bit word loaded into the data register should be zero (the phase registers are 12 bits wide).
To alter the entire contents of a frequency register, four write operations are needed. However, the 16 MSBs of a frequency word are contained in a separate register to the 16 LSBs. There­fore, the 16 MSBs of the frequency word can be altered inde­pendent of the 16 LSBs.
The phase and frequency registers to be used are selected using the pins FSELECT, PSEL0 and PSEL1 or the corresponding bits can be used. Bit SELSRC determines whether the bits or the pins are used. When SELSRC = 0, the pins are used while the bits are used when SELSRC = 1. When CLR is taken high, SELSRC is set to 0 so that the pins are the default source.
Data transfers from the serial (defer) register to the 16-bit data register, and the FSELECT and PSEL registers, occur following the 16th falling SCLK edge. Transfer of the data from the 16-bit data register to the destination register or from the FSELECT/ PSEL register to the respective multiplexer occurs on the next MCLK rising edge. Since the SCLK and the MCLK are asyn­chronous, an MCLK rising edge may occur while the data bits are in transitional state, which will cause a brief spurious DAC output if the register being written to is generating the DAC output. To avoid such spurious outputs, the AD9835 contains synchronizing circuitry. When the SYNC bit is set to 1, the synchronizer is enabled and data transfers from the serial
register (defer register) to the 16-bit data register and the FSELECT/PSEL registers occur following a two-stage pipeline delay which is triggered on the MCLK falling edge. The pipe­line delay ensures that the data is valid when the transfer occurs. Similarly, selection of the frequency/phase registers using the FSELECT/PSEL pins is synchronized with the MCLK rising edge when SYNC = 1. When SYNC = 0, the synchronizer is bypassed.
Selecting the frequency/phase registers using the pins is synchronized with MCLK internally also when SYNC = 1 to ensure that these inputs are valid at the MCLK rising edge. If times t
11
and t
11A
are met, the inputs will be at steady state at
the MCLK rising edge. However, if times t
11
and t
11A
are violated, the internal synchronizing circuitry will delay the instant at which the pins are sampled, ensuring that the inputs are valid at the sampling instant.
A latency is associated with each operation. When inputs FSELECT/PSEL change value, there will be a pipeline delay before control is transferred to the selected register—there will be a pipeline delay before the analog output is controlled by the selected register. When times t
11
and t
11A
are met, PSEL0, PSEL1 and FSELECT have latencies of six MCLK cycles when SYNC = 0. When SYNC = 1, the latency is increased to 8 MCLK cycles. When times t
11
and t
11A
are not met, the latency can increase by one MCLK cycle. Similarly, there is a latency associated with each write operation. If a selected frequency/phase register is loaded with a new word, there is a delay of 6 to 7 MCLK cycles before the analog output will change (there is an uncertainty of one MCLK cycle regarding the MCLK rising edge at which the data is loaded into the destination register). When SYNC = 1, the latency will be 8 or 9 MCLK cycles.
The flowchart in Figure 20 shows the operating routine for the AD9835. When the AD9835 is powered up, the part should be reset. This will reset the phase accumulator to zero so that the analog output is at full scale. To avoid spurious DAC outputs while the AD9835 is being initialized, the RESET bit should be set to 1 until the part is ready to begin generating an output. Taking CLR high will set SYNC and SELSRC to 0 so that the FSELECT/PSEL pins are used to select the frequency/phase registers and the synchronization circuitry is bypassed. A write operation is needed to the SYNC/SELSRC register to enable the synchronization circuitry or to change control to the FSELECT/PSEL bits. RESET does not reset the phase and frequency registers. These registers will contain invalid data and should therefore be set to a known value by the user. The RESET bit is then set to 0 to begin generating an output. A signal will appear at the DAC output 6 MCLK cycles after RESET is set to 0.
The analog output is f
MCLK
/2
32
× FREG where FREG is the
value loaded into the selected frequency register. This signal will be phase shifted by the amount specified in the selected
phase register (2 π/4096
× PHASEREG where PHASEREG is
the value contained in the selected phase register).
Control of the frequency/phase registers can be interchanged from the pins to the bits.
AD9835
–12– REV. 0
DATA WRITE**
FREG<0> = f
OUT0/fMCLK
*2
32
FREG<1> = f
OUT1/fMCLK
*2
32
PHASEREG <3:0> = DELTA PHASE<0, 1, 2, 3>
SELECT DATA SOURCES***
SET FSELECT
SET PSEL0, PSEL1
INITIALIZATION*
WAIT 6 MCLK CYCLES (8 MCLK CYCLES IF SYNC = 1)
DAC OUTPUT
V
OUT
= V
REFIN
*6.25*R
OUT/RSET
*(1 + COS(2p (FREG*f
MCLK
*t/232 + PHASEREG/212)))
NO
CHANGE f
OUT
?
YES
NO
YES
CHANGE FREG?
NO
YES
CHANGE FSELECT CHANGE PHASEREG?
NO
YES
CHANGE PSEL0, PSEL1
CHANGE PHASE?
Figure 20. Flowchart for AD9835 Initialization and Operation
INITIALIZATION*
CONTROL REGISTER WRITE
SET SLEEP
RESET = 1
CLR = 1
SET SYNC AND/OR SELSRC TO 1
YES
NO
CONTROL REGISTER WRITE
SYNC = 1
AND/OR
SELSRC = 1
WRITE INITIAL DATA
FREG<0> = f
OUT0/fMCLK
*2
32
FREG<1> = f
OUT1/fMCLK
*2
32
PHASEREG<3:0> = DELTA PHASE<0, 1, 2, 3>
SET PINS OR FREQUENCY/PHASE REGISTER WRITE
SET FSELECT, PSEL0 AND PSEL1
CONTROL REGISTER WRITE
SLEEP = 0 RESET = 0
CLR = 0
Figure 21. Initialization
DATA WRITE**
DEFERRED TRANSFER WRITE
WRITE 8 BITS TO DEFER REGISTER
DIRECT TRANSFER WRITE WRITE PRESENT 8 BITS AND 8 BITS IN DEFER REGISTER TO DATA REGISTER
WRITE ANOTHER WORD TO THIS
REGISTER?
WRITE A WORD TO ANOTHER REGISTER
CHANGE 8 BITS ONLY
YES
CHANGE 16 BITS
NOYES
NO
Figure 22. Data Writes
SELECT DATA SOURCES***
FSELECT/PSEL PINS BEING USED?
YES SELSRC = 0
SET PINS
SET FSELECT
SET PSEL0 SET PSEL1
FREQUENCY/PHASE REGISTER WRITE
SET FSELECT
SET PSEL0 SET PSEL1
SELSRC = 1
NO
Figure 23. Selecting Data Sources
AD9835
–13–REV. 0
APPLICATIONS
The AD9835 contains functions that make it suitable for modulation applications. The part can be used to perform simple modulation such as FSK. More complex modulation schemes such as GMSK and QPSK can also be implemented using the AD9835. In an FSK application, the two frequency registers of the AD9835 are loaded with different values; one frequency will represent the space frequency while the other will represent the mark frequency. The digital data stream is fed to the FSELECT pin, which will cause the AD9835 to modulate the carrier frequency between the two values.
The AD9835 has four phase registers; this enables the part to perform PSK. With phase shift keying, the carrier frequency is phase shifted, the phase being altered by an amount that is related to the bit stream being input to the modulator. The presence of four shift registers eases the interaction needed between the DSP and the AD9835.
The AD9835 is also suitable for signal generator applications. With its low current consumption, the part is suitable for applications in which it can be used as a local oscillator.
Grounding and Layout
The printed circuit board that houses the AD9835 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 which can be separated easily. A minimum etch technique is generally best for ground planes as it gives the best shielding. Digital and analog ground planes should only be joined in one place. If the AD9835 is the only device requiring an AGND to DGND connection, then the ground planes should be connected at the AGND and DGND pins of the AD9835. If the AD9835 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 AD9835.
Avoid running digital lines under the device as these will couple noise onto the die. The analog ground plane should be allowed to run under the AD9835 to avoid noise coupling. The power supply lines to the AD9835 should use as large a track as is 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. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This will 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 while signals are placed on the other side.
Good decoupling is important. The analog and digital supplies to the AD9835 are independent and separately pinned out to minimize coupling between 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
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 of the AD9835, it is recommended that the system’s AVDD supply be used. This supply should have the recommended analog supply decoupling between the AVDD pins of the AD9835 and AGND and the recommended digital supply decoupling capacitors between the DVDD pins and DGND.
Interfacing the AD9835 to Microprocessors
The AD9835 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/control informa­tion into the device. The serial clock can have a frequency of 20 MHz maximum. The serial clock can be continuous or it can idle high or low between write operations. When data/ control information is being written to the AD9835, FSYNC is taken low and held low while the 16 bits of data are being written into the AD9835. The FSYNC signal frames the 16 bits of information being loaded into the AD9835.
AD9835-to-ADSP-21xx Interface
Figure 24 shows the serial interface between the AD9835 and the ADSP-21xx. The ADSP-21xx should be set up to operate in the SPORT Transmit Alternate Framing Mode (TFSW = 1). The ADSP-21xx is programmed through the SPORT control register and should be configured as follows: Internal clock operation (ISCLK = 1), Active low framing (INVTFS = 1), 16-bit word length (SLEN = 15), Internal frame sync signal (ITFS = 1), Generate a frame sync for each write operation (TFSR = 1). Transmission is initiated by writing a word to the Tx register after the SPORT has been enabled. The data is clocked out on each rising edge of the serial clock and clocked into the AD9835 on the SCLK falling edge.
ADSP-2101/
ADSP-2103
AD9835
TFS
DT
SCLK
FSYNC
ADDITIONAL PINS OMITTED FOR CLARITY
SDATA SCLK
Figure 24. ADSP-2101/ADSP-2103-to-AD9835 Interface
AD9835-to-68HC11/68L11 Interface
Figure 25 shows the serial interface between the AD9835 and the 68HC11/68L11 microcontroller. The microcontroller is configured as the master by setting bit MSTR in the SPCR to 1 and, this provides a serial clock on SCK while the MOSI output drives the serial data line SDATA. Since 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: the SCK idles high between write operations (CPOL = 0), data is valid on the SCK falling edge (CPHA = 1). When data is being transmitted to the AD9835, the FSYNC line is taken low (PC7). Serial data from the 68HC11/68L11 is transmitted in 8-bit bytes with only eight
AD9835
–14– REV. 0
falling clock edges occurring in the transmit cycle. Data is transmitted MSB first. In order to load data into the AD9835, PC7 is held low after the first eight bits are transferred and a second serial write operation is performed to the AD9835. Only after the second eight bits have been transferred should FSYNC be taken high again.
68HC11/68L11
AD9835
PC7
MOSI
SCK
SDATA SCLK
FSYNC
ADDITIONAL PINS OMITTED FOR CLARITY
Figure 25. 68HC11/68L11-to-AD9835 Interface
AD9835-to-80C51/80L51 Interface
Figure 26 shows the serial interface between the AD9835 and the 80C51/80L51 microcontroller. The microcontroller is oper­ated in Mode 0 so that TXD of the 80C51/80L51 drives SCLK of the AD9835 while RXD drives the serial data line SDATA. The FSYNC signal is again derived from a bit programmable pin on the port (P3.3 being used in the diagram). When data is to be transmitted to the AD9835, P3.3 is taken low. The 80C51/80L51 transmits data in 8-bit bytes thus, only eight falling SCLK edges occur in each cycle. To load the remaining eight bits to the AD9835, P3.3 is held low after the first eight bits have been 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 which has the LSB first. The AD9835 accepts the MSB first (the 4 MSBs being the control information, the next 4 bits being the address while the 8 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
AD9835
P3.3 RXD
TXD
SDATA SCLK
FSYNC
ADDITIONAL PINS OMITTED FOR CLARITY
Figure 26. 80C51/80L51 to AD9835 Interface
AD9835-to-DSP56002 Interface
Figure 27 shows the interface between the AD9835 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 will frame the 16 bits (FSL = 0).
The frame sync signal is available on pin SC2 but, it needs to be inverted before being applied to the AD9835. The interface to the DSP56000/DSP56001 is similar to that of the DSP56002.
DSP56002
AD9835
SC2 STD
SCK
SDATA
SCLK
FSYNC
ADDITIONAL PINS OMITTED FOR CLARITY
Figure 27. AD9835-to-DSP56002 Interface
AD9835 Evaluation Board
The AD9835 Evaluation Board allows designers to evaluate the high performance AD9835 DDS Modulator with a minimum of effort.
To prove that this device will meet the user’s waveform synthe­sis requirements, the user only requires a 5 V power supply, an IBM-compatible PC and a spectrum analyzer along with the evaluation board. The evaluation setup is shown below.
The DDS Evaluation kit includes a populated, tested AD9835 printed circuit board along with the software that controls the AD9835 in a Windows
®
environment.
AD9835.EXE
IBM-COMPATIBLE PC
PARALLEL PORT
CENTRONICS
PRINTER CABLE
AD9835 EVALUATION
BOARD
Figure 28. AD9835 Evaluation Board Setup
Using the AD9835 Evaluation Board
The AD9835 Evaluation kit is a test system designed to simplify the evaluation of the AD9835. Provisions to control the AD9835 from the printer port of an IBM-compatible PC are included along with the necessary software. An application note is also available with the evaluation board which gives information on operating the evaluation board.
Prototyping Area
An area is available on the evaluation board where the user can add additional circuits to the evaluation test set. Users may want to build custom analog filters for the output or add buffers and operational amplifiers to be used in the final application.
XO vs. External Clock
The AD9835 can operate with master clocks up to 50 MHz. A 50 MHz oscillator is included on the evaluation board. How­ever, this oscillator can be removed and an external CMOS clock connected to the part, if required.
Power Supply
Power to the AD9835 Evaluation Board must be provided ex­ternally through the pin connections. The power leads should be twisted to reduce ground loops.
Windows is a registered trademark of Microsoft Corporation.
AD9835
–15–REV. 0
C1
0.1mF
DVDD AVDD
C2
0.1mF
415
DVDD AVDD
16 2
3
1
14
COMP REFIN
REFOUT
FSADJUST
IOUT
AVDD
C3 10nF
REFIN
C4 10nF
LK4
R5
3.9kV
IOUT
C8
10mFC70.1mF
DVDD
J2 J3
C9
0.1mF
C10 10mF
AVDD
R6
300V
DGND AGND
513
11 12 10
6
9
8
7
MCLK
FSELECT
PSEL0
PSEL1
FSYNC
SDATA
SCLK
16 14
9
20
11
6
4
DVDD
C6
0.1mF
110 19
U2
J1
SCLK SDATA FSYNC
R3 10kV
R1 10kV
R2 10kV
PSEL1
PSEL0
FSELECT
LK1
LK2
LK3
SW
DVDD
MCLK
R4 50V
DGND
DVDD
OUT
DVDD
C5
0.1mF
U3
SCLK SDATA
FSYNC
1 2 3
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
U1
AD9835
XTAL1
Figure 29. Evaluation Board Layout
Integrated Circuits
XTAL1 OSC XTAL 50 MHz U1 AD9835 (16-Lead TSSOP) U2 74HCT244 Buffer
Capacitors
C1, C2 0.1 µF Ceramic Chip Capacitor
C3, C4 10 nF Ceramic Capacitor
C5, C6, C7, C9 0.1 µF Ceramic Capacitor C8, C10 10 µF Tantalum Capacitor
Resistors
R1–R3 10 k Resistor R4 50 Resistor R5 3.9 k Resistor R6 300 Resistor
Links
LK1–LK3 Three Pin Link LK4 Two Pin Link
Switch
SW End Stackable Switch (SDC Double
Throw)
Sockets
MCLK, PSEL0, Subminiature BNC Connector PSEL1, FSELECT, IOUT, REFIN
Connectors
J1 36-Pin Edge Connector J2, J3 PCB Mounting Terminal Block
AD9835
–16–
REV. 0
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
C3309–8–7/98
PRINTED IN U.S.A.
16-Lead Thin Shrink Small Outline Package (TSSOP)
(RU-16)
16 9
8
1
0.201 (5.10)
0.193 (4.90)
0.256 (6.50)
0.246 (6.25)
0.177 (4.50)
0.169 (4.30)
PIN 1
SEATING
PLANE
0.006 (0.15)
0.002 (0.05)
0.0118 (0.30)
0.0075 (0.19)
0.0256 (0.65)
BSC
0.0433 (1.10) MAX
0.0079 (0.20)
0.0035 (0.090)
0.028 (0.70)
0.020 (0.50)
8° 0°
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