a
FEATURES
Member of Pin-Compatible TxDAC Product Family
125 MSPS Update Rate
14-Bit Resolution
Excellent SFDR and IMD
Differential Current Outputs: 2 mA to 20 mA
Power Dissipation: 190 mW @ 5 V to 45 mW @ 3 V
Power-Down Mode: 25 mW @ 5 V
On-Chip 1.20 V Reference
Single +5 V or +3 V Supply Operation
Packages: 28-Lead SOIC and TSSOP
Edge-Triggered Latches
APPLICATIONS
Communication Transmit Channel:
Basestations
ADSL/HFC Modems
Instrumentation
PRODUCT DESCRIPTION
The AD9764 is the 14-bit resolution member of the TxDAC
series of high performance, low power CMOS digital-to-analog
converters (DACs). The TxDAC family, which consists of pin
compatible 8-, 10-, 12-, and 14-bit DACs, is specifically optimized for the transmit signal path of communication systems.
All of the devices share the same interface options, small outline
package and pinout, providing an upward or downward component selection path based on performance, resolution and cost.
The AD9764 offers exceptional ac and dc performance while
supporting update rates up to 125 MSPS.
The AD9764’s flexible single-supply operating range of 2.7 V to
5.5 V and low power dissipation are well suited for portable and
low power applications. Its power dissipation can be further
reduced to a mere 45 mW with a slight degradation in performance
by lowering the full-scale current output. Also, a power-down
mode reduces the standby power dissipation to approximately
25 mW.
The AD9764 is manufactured on an advanced CMOS process.
A segmented current source architecture is combined with a
proprietary switching technique to reduce spurious components
and enhance dynamic performance. Edge-triggered input
latches and a 1.2 V temperature compensated bandgap reference have been integrated to provide a complete monolithic
DAC solution. Flexible supply options support +3 V and +5 V
CMOS logic families.
The AD9764 is a current-output DAC with a nominal full-scale
output current of 20 mA and > 100 kΩ output impedance.
14-Bit, 125 MSPS
TxDAC® D/A Converter
AD9764*
FUNCTIONAL BLOCK DIAGRAM
+5V
0.1mF
REFLO
COMP1
AVDD
+1.20V REF
0.1mF
CURRENT
SOURCE
ARRAY
FS ADJ
RSET
+5V
DVDD
DCOM
CLOCK
AD9764
50pF
REFIO
ACOM
COMP2
0.1mF
IOUTA
SEGMENTED
SWITCHES
CLOCK
LSB
SWITCHES
IOUTB
LATCHES
SLEEP
DIGITAL DATA INPUTS (DB13–DB0)
Differential current outputs are provided to support singleended or differential applications. Matching between the two
current outputs ensures enhanced dynamic performance in a
differential output configuration. The current outputs may be
tied directly to an output resistor to provide two complementary, single-ended voltage outputs or fed directly into a transformer. The output voltage compliance range is 1.25 V.
The on-chip reference and control amplifier are configured for
maximum accuracy and flexibility. The AD9764 can be driven
by the on-chip reference or by a variety of external reference
voltages. The internal control amplifier, which provides a wide
(>10:1) adjustment span, allows the AD9764 full-scale current
to be adjusted over a 2 mA to 20 mA range while maintaining
excellent dynamic performance. Thus, the AD9764 may operate
at reduced power levels or be adjusted over a 20 dB range to
provide additional gain ranging capabilities.
The AD9764 is available in 28-lead SOIC and TSSOP packages.
It is specified for operation over the industrial temperature range.
PRODUCT HIGHLIGHTS
REV. B
1. The AD9764 is a member of the TxDAC product family that
provides an upward or downward component selection path
based on resolution (8 to 14 bits), performance and cost.
2. Manufactured on a CMOS process, the AD9764 uses a proprietary switching technique that enhances dynamic performance beyond that previously attainable by higher power/cost
bipolar or BiCMOS devices.
3. On-chip, edge-triggered input CMOS latches readily interface
to +3 V and +5 V CMOS logic families. The AD9764 can
support update rates up to 125 MSPS.
4. A flexible single-supply operating range of 2.7 V to 5.5 V, and
a wide full-scale current adjustment span of 2 mA to 20 mA,
allows the AD9764 to operate at reduced power levels.
5. The current output(s) of the AD9764 can be easily configured for various single-ended or differential circuit topologies.
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
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., 1999
TxDAC is a registered trademark of Analog Devices, Inc.
*Patent pending.
AD9764–SPECIFICATIONS
DC SPECIFICATIONS (T
MIN
to TMAX , AVDD = +5 V, DVDD = +5 V, IOUTFS = 20 mA, unless otherwise noted)
Parameter
Min
RESOLUTION
Typ
Max
14
Units
Bits
1
DC ACCURACY
Integral Linearity Error (INL)
TA = +25°C
TMIN to TMAX
Differential Nonlinearity (DNL)
TA = +25°C
TMIN to TMAX
ANALOG OUTPUT
Offset Error
Gain Error (Without Internal Reference)
Gain Error (With Internal Reference)
Full-Scale Output Current2
Output Compliance Range
Output Resistance
Output Capacitance
REFERENCE OUTPUT
Reference Voltage
Reference Output Current3
REFERENCE INPUT
Input Compliance Range
Reference Input Resistance
Small Signal Bandwidth (w/o CCOMP1)4
–4.5
–6.5
± 2.5
+4.5
+6.5
LSB
LSB
–2.5
–4.5
± 1.5
+2.5
+4.5
LSB
LSB
+0.025
+2
+7
20.0
1.25
% of FSR
% of FSR
% of FSR
mA
V
kΩ
pF
1.32
V
nA
1.25
1
1.4
V
MΩ
MHz
0
± 50
± 100
± 50
ppm of FSR/°C
ppm of FSR/°C
ppm of FSR/°C
ppm/°C
–0.025
–2
–7
2.0
–1.0
100
5
1.08
1.20
100
0.1
TEMPERATURE COEFFICIENTS
Offset Drift
Gain Drift (Without Internal Reference)
Gain Drift (With Internal Reference)
Reference Voltage Drift
POWER SUPPLY
Supply Voltages
AVDD5
DVDD
Analog Supply Current (IAVDD )
Digital Supply Current (IDVDD)6
Supply Current Sleep Mode (IAVDD)
Power Dissipation6 (5 V, IOUTFS = 20 mA)
Power Dissipation7 (5 V, IOUTFS = 20 mA)
Power Dissipation7 (3 V, IOUTFS = 2 mA)
Power Supply Rejection Ratio8—AVDD
Power Supply Rejection Ratio8—DVDD
OPERATING RANGE
±1
±1
2.7
2.7
5.0
5.0
25
1.5
5.0
133
190
45
5.5
5.5
30
4
8.5
170
–0.4
–0.025
+0.4
+0.025
V
V
mA
mA
mA
mW
mW
mW
% of FSR/V
% of FSR/V
–40
+85
°C
NOTES
1
Measured at IOUTA , driving a virtual ground.
2
Nominal full-scale current, I OUTFS, is 32 × the I REF current.
3
Use an external buffer amplifier to drive any external load.
4
Reference bandwidth is a function of external cap at COMP1 pin and signal level.
5
For operation below 3 V, it is recommended that the output current be reduced to 12 mA or less to maintain optimum performance.
6
Measured at fCLOCK = 25 MSPS and fOUT = 1.0 MHz.
7
Measured as unbuffered voltage output with I OUTFS = 20 mA and 50 Ω R LOAD at IOUTA and IOUTB, fCLOCK = 100 MSPS and fOUT = 40 MHz.
8
± 5% Power supply variation.
Specifications subject to change without notice.
–2–
REV. B
AD9764
(TMIN to TMAX , AVDD = +5 V, DVDD = +5 V, IOUTFS = 20 mA, Differential Transformer Coupled Output,
DYNAMIC SPECIFICATIONS 50 ⍀ Doubly Terminated, unless otherwise noted)
Parameter
Min
DYNAMIC PERFORMANCE
Maximum Output Update Rate (fCLOCK)
Output Settling Time (tST) (to 0.1%)1
Output Propagation Delay (tPD)
Glitch Impulse
Output Rise Time (10% to 90%)1
Output Fall Time (10% to 90%)1
Output Noise (IOUTFS = 20 mA)
Output Noise (IOUTFS = 2 mA)
Max
125
75
73
82
dBc
dBc
dBc
dBc
dBc
dBc
dBc
dBc
dBc
85
77
70
80
77
70
58
78
76
89
dBc
dBc
dBc
dBc
84
84
–78
NOTES
1
Measured single-ended into 50 Ω load.
Specifications subject to change without notice.
–3–
Units
MSPS
ns
ns
pV-s
ns
ns
pA/√Hz
pA/√Hz
35
1
5
2.5
2.5
50
30
AC LINEARITY
Spurious-Free Dynamic Range to Nyquist
fCLOCK = 25 MSPS; fOUT = 1.00 MHz
0 dBFS Output
TA = +25°C
TMIN to TMAX
–6 dBFS Output
–12 dBFS Output
–18 dBFS Output
fCLOCK = 50 MSPS; fOUT = 1.00 MHz
fCLOCK = 50 MSPS; fOUT = 2.51 MHz
fCLOCK = 50 MSPS; fOUT = 5.02 MHz
fCLOCK = 50 MSPS; fOUT = 20.2 MHz
Spurious-Free Dynamic Range within a Window
fCLOCK = 25 MSPS; fOUT = 1.00 MHz; 2 MHz Span
TA = +25°C
TMIN to TMAX
fCLOCK = 50 MSPS; fOUT = 5.02 MHz; 2 MHz Span
fCLOCK = 100 MSPS; fOUT = 5.04 MHz; 4 MHz Span
Total Harmonic Distortion
fCLOCK = 25 MSPS; fOUT = 1.00 MHz
TA = +25°C
TMIN to TMAX
fCLOCK = 50 MHz; fOUT = 2.00 MHz
fCLOCK = 100 MHz; fOUT = 2.00 MHz
Multitone Power Ratio (Eight Tones at 110 kHz Spacing)
fCLOCK = 20 MSPS; fOUT = 2.00 MHz to 2.99 MHz
0 dBFS Output
–6 dBFS Output
–12 dBFS Output
–18 dBFS Output
REV. B
Typ
–74
–72
–75
–75
dBc
dBc
dBc
dBc
73
76
73
64
dBc
dBc
dBc
dBc
AD9764
DIGITAL SPECIFICATIONS
Parameter
DIGITAL INPUTS
Logic “1” Voltage @ DVDD = +5 V
Logic “1” Voltage @ DVDD = +3 V
Logic “0” Voltage @ DVDD = +5 V
Logic “0” Voltage @ DVDD = +3 V
Logic “1” Current
Logic “0” Current
Input Capacitance
Input Setup Time (tS)
Input Hold Time (tH)
Latch Pulsewidth (tLPW)
Min
Typ
3.5
2.1
5
3
0
0
Max
Units
V
V
V
V
µA
µA
pF
ns
ns
ns
1.3
0.9
+10
+10
–10
–10
5
2.0
1.5
3.5
Specifications subject to change without notice.
DB0–DB13
tS
tH
CLOCK
tLPW
tPD
tST
IOUTA
OR
IOUTB
0.1%
0.1%
Figure 1. Timing Diagram
ABSOLUTE MAXIMUM RATINGS*
Parameter
AVDD
DVDD
ACOM
AVDD
CLOCK, SLEEP
Digital Inputs
IOUTA , IOUTB
COMP1, COMP2
REFIO, FSADJ
REFLO
Junction Temperature
Storage Temperature
Lead Temperature
(10 sec)
ORDERING GUIDE
With
Respect to
Min
Max
Units
ACOM
DCOM
DCOM
DVDD
DCOM
DCOM
ACOM
ACOM
ACOM
ACOM
–0.3
–0.3
–0.3
–6.5
–0.3
–0.3
–1.0
–0.3
–0.3
–0.3
+6.5
+6.5
+0.3
+6.5
DVDD + 0.3
DVDD + 0.3
AVDD + 0.3
AVDD + 0.3
AVDD + 0.3
+0.3
+150
+150
V
V
V
V
V
V
V
V
V
V
°C
°C
+300
°C
–65
Model
Temperature
Range
Package
Description
Package
Options*
AD9764AR –40°C to +85°C 28-Lead 300 mil SOIC R-28
AD9764ARU –40°C to +85°C 28-Lead TSSOP
RU-28
AD9764-EB Evaluation Board
*R = Small Outline IC, RU = TSSOP.
THERMAL CHARACTERISTICS
Thermal Resistance
28-Lead 300 mil SOIC
θJA = 71.4°C/W
θJC = 23°C/W
28-Lead TSSOP
θJA = 97.9°C/W
θJC = 14.0°C/W
*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
sections of this specification is not implied. Exposure to absolute maximum ratings
for extended periods may affect device reliability.
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD9764 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
–4–
WARNING!
ESD SENSITIVE DEVICE
REV. B
AD9764
PIN CONFIGURATION
(MSB) DB13 1
28 CLOCK
DB12 2
27 DVDD
DB11 3
26 DCOM
DB10 4
25 NC
DB9 5
AD9764
24 AVDD
DB8 6
TOP VIEW 23 COMP2
DB7 7 (Not to Scale) 22 IOUTA
DB6 8
21 IOUTB
DB5 9
20 ACOM
DB4 10
19 COMP1
DB3 11
18 FS ADJ
DB2 12
17 REFIO
DB1 13
16 REFLO
DB0 14
15 SLEEP
NC = NO CONNECT
PIN FUNCTION DESCRIPTIONS
Pin No.
Name
Description
1
2–13
14
15
DB13
DB12–DB1
DB0
SLEEP
16
17
REFLO
REFIO
18
19
20
21
22
23
24
25
26
27
28
FS ADJ
COMP1
ACOM
IOUTB
IOUTA
COMP2
AVDD
NC
DCOM
DVDD
CLOCK
Most Significant Data Bit (MSB).
Data Bits 1–12.
Least Significant Data Bit (LSB).
Power-Down Control Input. Active High. Contains active pull-down circuit; it may be left unterminated if
not used.
Reference Ground when Internal 1.2 V Reference Used. Connect to AVDD to disable internal reference.
Reference Input/Output. Serves as reference input when internal reference disabled (i.e., Tie REFLO to
AVDD). Serves as 1.2 V reference output when internal reference activated (i.e., Tie REFLO to ACOM).
Requires 0.1 µF capacitor to ACOM when internal reference activated.
Full-Scale Current Output Adjust.
Bandwidth/Noise Reduction Node. Add 0.1 µF to AVDD for optimum performance.
Analog Common.
Complementary DAC Current Output. Full-scale current when all data bits are 0s.
DAC Current Output. Full-scale current when all data bits are 1s.
Internal Bias Node for Switch Driver Circuitry. Decouple to ACOM with 0.1 µF capacitor.
Analog Supply Voltage (+2.7 V to +5.5 V).
No Internal Connection.
Digital Common.
Digital Supply Voltage (+2.7 V to +5.5 V).
Clock Input. Data latched on positive edge of clock.
REV. B
–5–
AD9764
DEFINITIONS OF SPECIFICATIONS
Linearity Error (Also Called Integral Nonlinearity or INL)
Power Supply Rejection
The maximum change in the full-scale output as the supplies
are varied over a specified range.
Linearity error is defined as the maximum deviation of the
actual analog output from the ideal output, determined by a
straight line drawn from zero to full scale.
Settling Time
The time required for the output to reach and remain within a
specified error band about its final value, measured from the
start of the output transition.
Differential Nonlinearity (or DNL)
DNL is the measure of the variation in analog value, normalized
to full scale, associated with a 1 LSB change in digital input
code.
Glitch Impulse
Asymmetrical switching times in a DAC give rise to undesired
output transients that are quantified by a glitch impulse. It is
specified as the net area of the glitch in pV-s.
Offset Error
The deviation of the output current from the ideal of zero is
called offset error. For IOUTA, 0 mA output is expected when the
inputs are all 0s. For IOUTB, 0 mA output is expected when all
inputs are set to 1s.
Spurious-Free Dynamic Range
The difference, in dB, between the rms amplitude of the output
signal and the peak spurious signal over the specified bandwidth.
Gain Error
Total Harmonic Distortion
The difference between the actual and ideal output span. The
actual span is determined by the output when all inputs are set
to 1s minus the output when all inputs are set to 0s.
THD is the ratio of the sum of the rms value of the first six
harmonic components to the rms value of the measured output
signal. It is expressed as a percentage or in decibels (dB).
Output Compliance Range
Multitone Power Ratio
The range of allowable voltage at the output of a current-output
DAC. Operation beyond the maximum compliance limits may
cause either output stage saturation or breakdown, resulting in
nonlinear performance.
The spurious-free dynamic range for an output containing multiple carrier tones of equal amplitude. It is measured as the
difference between the rms amplitude of a carrier tone to the
peak spurious signal in the region of a removed tone.
Temperature Drift
Temperature drift is specified as the maximum change from the
ambient (+25°C) value to the value at either TMIN or TMAX . For
offset and gain drift, the drift is reported in ppm of full-scale
range (FSR) per degree C. For reference drift, the drift is
reported in ppm per degree C.
+5V
0.1mF
REFLO
+1.20V REF
0.1mF
REFIO
+5V
RETIMED
CLOCK
OUTPUT*
LECROY 9210
PULSE GENERATOR
50pF
COMP2
0.1mF
MINI-CIRCUITS
T1-1T
IOUTA
SEGMENTED SWITCHES
FOR DB13–DB5
CLOCK
50V
ACOM
AD9764
DVDD
DCOM
DVDD
DCOM
AVDD
PMOS
CURRENT SOURCE
ARRAY
FS ADJ
RSET
2kV
COMP1
LSB
SWITCHES
IOUTB
100V
TO HP3589A
SPECTRUM/
NETWORK
ANALYZER
50V INPUT
LATCHES
50V
SLEEP
50V
CLOCK
OUTPUT
DIGITAL
DATA
20pF
20pF
* AWG2021 CLOCK RETIMED
SUCH THAT DIGITAL DATA
TRANSITIONS ON FALLING EDGE
OF 50% DUTY CYCLE CLOCK.
TEKTRONIX
AWG-2021
Figure 2. Basic AC Characterization Test Setup
–6–
REV. B
AD9764
Typical AC Characterization Curves
(AVDD = +5 V, DVDD = +3 V, IOUTFS = 20 mA, 50 ⍀ Doubly Terminated Load, Differential Output, TA = +25ⴗC, SFDR up to Nyquist, unless otherwise noted)
90
85
85
5 MSPS
80
25 MSPS
SFDR – dBc
100 MSPS
60
75
–12dBFS
70
65
60
70
65
55
55
50
50
50
45
45
45
40
0.1
40
100
90
85
–6dBFS
2.5
0
SFDR – dBc
0dBFS
Figure 5. SFDR vs. fOUT @ 25 MSPS
85
–6dBFS
60
50
50
45
45
40
–12dBFS
65
55
0dBFS
10mA @ 5V
20mA @ 5V
5mA @ 5V
25
Figure 6. SFDR vs. fOUT @ 50 MSPS
70
65
20mA @ 3V
5mA @ 3V
10mA @ 3V
55
0
10
20
30
40
FREQUENCY – MHz
50
0.0
50
Figure 7. SFDR vs. fOUT @100 MSPS
90
75
60
40
10
15
20
FREQUENCY – MHz
12
10
90
70
55
5
4
6
8
FREQUENCY – MHz
75
65
0
2
80
70
60
1.0
1.5
2.0
FREQUENCY – MHz
80
–12dBFS
75
0.5
Figure 4. SFDR vs. fOUT @ 5 MSPS
85
80
40
0
90
0dBFS
60
55
10
1
FREQUENCY – MHz
–12dBFS
80
SFDR – dBc
SFDR – dBc
65
Figure 3. SFDR vs. f OUT @ 0 dBFS
SFDR – dBc
0dBFS
75
50 MSPS
70
–6dBFS
85
80
75
90
–6dBFS
SFDR – dBc
90
2.0
4.0
6.0
8.0
FREQUENCY – MHz
10.0
Figure 8. SFDR vs. f OUT and
IOUTFS @ 25 MSPS and 0 dBFS
90
90
455kHz @ 5 MSPS
80
9.09MHz @ 100 MSPS
SFDR – dBc
80
70
60
3.38/3.63MHz @ 25 MSPS
1MHz @ 5 MSPS
2.27MHz @ 25 MSPS
SFDR – dBc
SFDR – dBc
80
5MHz @ 25 MSPS
70
0.675/0.725MHz @ 5 MSPS
70
60
60
13.5/14.5MHz @ 100 MSPS
20MHz @ 100 MSPS
4.55MHz @ 50 MSPS
10MHz @ 50 MSPS
50
–30
–25
–20
–15
–10
AOUT – dBFS
–5
0
Figure 9. Single-Tone SFDR vs. AOUT
@ fOUT = fCLOCK/11
REV. B
50
–30
–25
–20
–15
–10
AOUT – dBFS
6.75/7.25 @ 50 MSPS
–5
0
Figure 10. Single-Tone SFDR vs.
AOUT @ fOUT = fCLOCK/5
–7–
50
–30
–25
–20
–15
–10
AOUT – dBFS
–5
0
Figure 11. Dual-Tone SFDR vs. AOUT
@ fOUT = fCLOCK/7
AD9764
–70
80
85
80
2ND HARMONIC
SNR – dB
dBc
–80
3RD HARMONIC
–85
4TH HARMONIC
–90
–95
000.0E+0
40.0E+6
80.0E+6
IOUTFS = 10mA,
DVDD = +3V
IOUTFS = 10mA,
DVDD = +5V
70
Figure 13. SNR vs. fCLOCK @ fOUT =
2.0 MHz
2.0
1.5
70
IDIFF @ 0dBFS
IA @ 0dBFS
50
1
10
OUTPUT FREQUENCY – MHz
2.0
80
1.5
75
–0.5
1.0
SFDR – dBc
ERROR – LSB
ERROR – LSB
0.0
0.5
0.0
100
Figure 14. Differential vs. SingleEnded SFDR vs. fOUT @ 50 MSPS
2.5MHz
1.0
0.5
IA @ –6dBFS
60
65 IOUTFS = 5mA,
DVDD = +3V
IOUTFS = 5mA,
DVDD = +5V
60
0 10 20 30 40 50 60 70 80 90 100
fCLOCK – MSPS
120.0E+6
Figure 12. THD vs. f CLOCK @
fOUT = 2 MHz
75
SFDR – dBc
–75
IDIFF @ –6dBFS
IOUTFS = 20mA,
DVDD = +3V
IOUTFS = 20mA,
DVDD = +5V
70
65
10MHz
60
–1.0
–0.5
55
–1.0
50
–40
–1.5
40MHz
4000
8000
CODE
12000
0
16000
Figure 15. Typical INL
8000
CODE
12000
16000
Figure 16. Typical DNL
0
–20
–20
–30
10dB – Div
–40
–50
–40
–50
–60
–60
–70
–70
–80
–80
–90
000.0E+0
–90
0E+0
7.5E+6
15.0E+6
22.5E+6
Figure 18. Single-Tone SFDR
0
20
40
60
TEMPERATURE – 8C
80
0
fCLK = 50MSPS
fOUT1 = 6.75MHz
fOUT2 = 7.25MHz
SFDR = 69dBc
AMPLITUDE = 0dBFS
–10
–30
–20
Figure 17. SFDR vs. Temperature
@ 100 MSPS, 0 dBFS
0
fCLK = 50MSPS
fOUT = 1.25MHz
SFDR = 78dBc
AMPLITUDE = 0dBFS
–10
10dB – Div
4000
fCLK = 50MSPS
fOUT1 = 6.25MHz
fOUT2 = 6.75MHz
fOUT3 = 7.25MHz
fOUT4 = 7.75MHz
SFDR = 66dBc
AMPLITUDE = 0dBFS
–10
–20
–30
10dB – Div
–2.0
0
–40
–50
–60
–70
–80
–90
5E+6
10E+6
15E+6 20E+6
25E+6
Figure 19. Dual-Tone SFDR
–8–
–100
000.0E+0
7.5E+6
15.0E+6
22.5E+6
Figure 20. Four-Tone SFDR
REV. B
AD9764
+5V
0.1mF
REFLO
+1.20V REF
VREFIO
0.1mF
RSET
2kV
COMP1
FS ADJ
+5V
DVDD
DCOM
CLOCK
CLOCK
ACOM
AD9764
50pF
REFIO
IREF
AVDD
PMOS
CURRENT SOURCE
ARRAY
COMP2
0.1mF
VDIFF = VOUTA – VOUTB
IOUTA
LSB
SWITCHES
SEGMENTED SWITCHES
FOR DB13–DB5
IOUTB
IOUTA
IOUTB
VOUTA
VOUTB
RLOAD
50V
LATCHES
SLEEP
RLOAD
50V
DIGITAL DATA INPUTS (DB13–DB0)
Figure 21. Functional Block Diagram
IOUTB = (16383 – DAC CODE)/16384 × IOUTFS
FUNCTIONAL DESCRIPTION
Figure 21 shows a simplified block diagram of the AD9764. The
AD9764 consists of a large PMOS current source array that is
capable of providing up to 20 mA of total current. The array
is divided into 31 equal currents that make up the five most
significant bits (MSBs). The next four bits or middle bits consist
of 15 equal current sources whose value is 1/16th of an MSB
current source. The remaining LSBs are binary weighted fractions of the middle bits current sources. Implementing the
middle and lower bits with current sources, instead of an R-2R
ladder, enhances its dynamic performance for multitone or low
amplitude signals and helps maintain the DAC’s high output
impedance (i.e., >100 kΩ).
All of these current sources are switched to one or the other of
the two output nodes (i.e., IOUTA or IOUTB) via PMOS differential current switches. The switches are based on a new architecture that drastically improves distortion performance. This new
switch architecture reduces various timing errors and provides
matching complementary drive signals to the inputs of the differential current switches.
As mentioned previously, IOUTFS is a function of the reference
current IREF, which is nominally set by a reference voltage
VREFIO and external resistor RSET. It can be expressed as:
IOUTFS = 32 × IREF
where IREF = VREFIO /R SET
(3)
(4)
The two current outputs will typically drive a resistive load
directly or via a transformer. If dc coupling is required, IOUTA
and IOUTB should be directly connected to matching resistive
loads, RLOAD, that are tied to analog common, ACOM. Note
that RLOAD may represent the equivalent load resistance seen by
IOUTA or IOUTB as would be the case in a doubly terminated
50 Ω or 75 Ω cable. The single-ended voltage output appearing
at the IOUTA and IOUTB nodes is simply:
VOUTA = IOUTA × RLOAD
(5)
VOUTB = IOUTB × R LOAD
(6)
The differential voltage, VDIFF , appearing across IOUTA and
IOUTB is:
VDIFF = (IOUTA – IOUTB) × R LOAD
(7)
Substituting the values of IOUTA , IOUTB and IREF; VDIFF can be
expressed as:
VDIFF = {(2 DAC CODE – 16383)/16384} ×
VDIFF = {(32 R LOAD/R SET) × VREFIO
The full-scale output current is regulated by the reference control amplifier and can be set from 2 mA to 20 mA via an external resistor, RSET. The external resistor, in combination with
both the reference control amplifier and voltage reference
VREFIO, sets the reference current I REF, which is mirrored over to
the segmented current sources with the proper scaling factor.
The full-scale current, IOUTFS, is 32 times the value of IREF.
(8)
These last two equations highlight some of the advantages of
operating the AD9764 differentially. First, the differential operation will help cancel common-mode error sources associated
with IOUTA and I OUTB such as noise, distortion and dc offsets.
Second, the differential code-dependent current and subsequent
voltage, VDIFF , is twice the value of the single-ended voltage
output (i.e., VOUTA or VOUTB), thus providing twice the signal
power to the load.
DAC TRANSFER FUNCTION
The AD9764 provides complementary current outputs, IOUTA
and IOUTB. IOUTA will provide a near full-scale current output,
IOUTFS, when all bits are high (i.e., DAC CODE = 16383) while
IOUTB, the complementary output, provides no current. The
current output appearing at IOUTA and IOUTB is a function of
both the input code and IOUTFS and can be expressed as:
REV. B
where DAC CODE = 0 to 16383 (i.e., Decimal Representation).
Note that the full-scale value of VOUTA and VOUTB should not
exceed the specified output compliance range to maintain specified distortion and linearity performance.
The analog and digital sections of the AD9764 have separate
power supply inputs (i.e., AVDD and DVDD) that can operate
independently over a 2.7 volt to 5.5 volt range. The digital
section, which is capable of operating up to a 125 MSPS clock
rate, consists of edge-triggered latches and segment decoding
logic circuitry. The analog section includes the PMOS current
sources, the associated differential switches, a 1.20 V bandgap
voltage reference and a reference control amplifier.
IOUTA = (DAC CODE/16384) × IOUTFS
(2)
(1)
–9–
Note that the gain drift temperature performance for a singleended (VOUTA and VOUTB) or differential output (VDIFF) of the
AD9764 can be enhanced by selecting temperature tracking
resistors for RLOAD and RSET due to their ratiometric relationship as shown in Equation 8.
AD9764
provides several application benefits. The first benefit relates
directly to the power dissipation of the AD9764, which is proportional to IOUTFS (refer to the Power Dissipation section). The
second benefit relates to the 20 dB adjustment, which is useful
for system gain control purposes.
REFERENCE OPERATION
The AD9764 contains an internal 1.20 V bandgap reference
that can be easily disabled and overridden by an external
reference. REFIO serves as either an input or output, depending
on whether the internal or external reference is selected. If
REFLO is tied to ACOM, as shown in Figure 22, the internal
reference is activated, and REFIO provides a 1.20 V output. In
this case, the internal reference must be compensated externally
with a ceramic chip capacitor of 0.1 µF or greater from REFIO
to REFLO. Also, REFIO should be buffered with an external
amplifier having an input bias current less than 100 nA if any
additional loading is required.
The small signal bandwidth of the reference control amplifier is
approximately 1.4 MHz and can be reduced by connecting an
external capacitor between COMP1 and AVDD. The output of
the control amplifier, COMP1, is internally compensated via a
50 pF capacitor that limits the control amplifier small-signal
bandwidth and reduces its output impedance. Any additional
external capacitance further limits the bandwidth and acts as a
filter to reduce the noise contribution from the reference amplifier. Figure 24 shows the relationship between the external
capacitor and the small signal –3 dB bandwidth of the reference amplifier. Since the –3 dB bandwidth corresponds to the
dominant pole, and hence the time constant, the settling time of
the control amplifier to a stepped reference input response can
be approximated.
+5V
0.1mF
OPTIONAL
EXTERNAL
REF BUFFER
REFLO
+1.2V REF
COMP1
50pF
REFIO
ADDITIONAL
LOAD
0.1mF
2kV
AVDD
CURRENT
SOURCE
ARRAY
FS ADJ
AD9764
1000
Figure 22. Internal Reference Configuration
BANDWIDTH – kHz
The internal reference can be disabled by connecting REFLO to
AVDD. In this case, an external reference may then be applied
to REFIO as shown in Figure 23. The external reference may
provide either a fixed reference voltage to enhance accuracy and
drift performance or a varying reference voltage for gain control.
Note that the 0.1 µF compensation capacitor is not required
since the internal reference is disabled, and the high input impedance (i.e., 1 MΩ) of REFIO minimizes any loading of the
external reference.
0.1
0.1
AVDD
0.1mF
REFLO
AVDD
COMP1
EXTERNAL
REF
FS ADJ
RSET
IREF =
VREFIO/RSET
AD9764
10
100
COMP1 CAPACITOR – nF
1000
The optimum distortion performance for any reconstructed
waveform is obtained with a 0.1 µF external capacitor installed.
Thus, if IREF is fixed for an application, a 0.1 µF ceramic chip
capacitor is recommended. Also, since the control amplifier is
optimized for low power operation, multiplying applications
requiring large signal swings should consider using an external
control amplifier to enhance the application’s overall large signal
multiplying bandwidth and/or distortion performance.
AVDD
50pF
REFIO
1
Figure 24. External COMP1 Capacitor vs. –3 dB Bandwidth
+1.2V REF
VREFIO
10
CURRENT
SOURCE
ARRAY
REFERENCE
CONTROL
AMPLIFIER
Figure 23. External Reference Configuration
REFERENCE CONTROL AMPLIFIER
The AD9764 also contains an internal control amplifier that is
used to regulate the DAC’s full-scale output current, IOUTFS.
The control amplifier is configured as a V-I converter, as shown
in Figure 23, such that its current output, IREF, is determined by
the ratio of the VREFIO and an external resistor, RSET, as stated
in Equation 4. IREF is copied over to the segmented current
sources with the proper scaling factor to set IOUTFS as stated in
Equation 3.
There are two methods in which IREF can be varied for a fixed
RSET. The first method is suitable for a single-supply system in
which the internal reference is disabled, and the common-mode
voltage of REFIO is varied over its compliance range of 1.25 V
to 0.10 V. REFIO can be driven by a single-supply amplifier or
DAC, thus allowing IREF to be varied for a fixed RSET. Since the
input impedance of REFIO is approximately 1 MΩ, a simple,
low cost R-2R ladder DAC configured in the voltage mode
topology may be used to control the gain. This circuit is shown
in Figure 25 using the AD7524 and an external 1.2 V reference,
the AD1580.
The control amplifier allows a wide (10:1) adjustment span of
IOUTFS over a 2 mA to 20 mA range by setting IREF between
62.5 µA and 625 µA. The wide adjustment span of IOUTFS
–10–
REV. B
AD9764
AVDD
OPTIONAL
BANDLIMITING
CAPACITOR
AVDD
REFLO
RFB
1.2V
0.1V TO 1.2V
VREF
OUT2
AD1580
REFIO
CURRENT
SOURCE
ARRAY
FS ADJ
AGND
AVDD
50pF
OUT1
AD7524
COMP1
+1.2V REF
VDD
RSET
IREF =
VREF/RSET
AD9764
DB7–DB0
Figure 25. Single-Supply Gain Control Circuit
The second method may be used in a dual-supply system in
which the common-mode voltage of REFIO is fixed, and IREF is
varied by an external voltage, VGC, applied to RSET via an amplifier. An example of this method is shown in Figure 26 in which
the internal reference is used to set the common-mode voltage
of the control amplifier to 1.20 V. The external voltage, VGC, is
referenced to ACOM and should not exceed 1.2 V. The value of
RSET is such that IREFMAX and IREFMIN do not exceed 62.5 µA
and 625 µA, respectively. The associated equations in Figure 26
can be used to determine the value of RSET.
OPTIONAL
BANDLIMITING
CAPACITOR
REFLO
COMP1 AVDD
50pF
CURRENT
SOURCE
ARRAY
FS ADJ
1mF
RSET
IREF
AD9764
IREF = (1.2–VGC)/RSET
WITH VGC < VREFIO AND 62.5mA # IREF # 625A
VGC
The AD9764 produces two complementary current outputs,
IOUTA and I OUTB, which may be configured for single-end
or differential operation. IOUTA and I OUTB can be converted into
complementary single-ended voltage outputs, VOUTA and
VOUTB, via a load resistor, R LOAD, as described in the DAC
Transfer Function section by Equations 5 through 8. The
differential voltage, VDIFF, existing between VOUTA and V OUTB
can also be converted to a single-ended voltage via a transformer
or differential amplifier configuration.
Figure 28 shows the equivalent analog output circuit of the
AD9764 consisting of a parallel combination of PMOS differential current switches associated with each segmented current
source. The output impedance of IOUTA and IOUTB is determined
by the equivalent parallel combination of the PMOS switches
and is typically 100 kΩ in parallel with 5 pF. Due to the nature of a PMOS device, the output impedance is also slightly
dependent on the output voltage (i.e., VOUTA and VOUTB) and, to
a lesser extent, the analog supply voltage, AVDD, and full-scale
current, IOUTFS. Although the output impedance’s signal dependency can be a source of dc nonlinearity and ac linearity (i.e.,
distortion), its effects can be limited if certain precautions are
noted.
AVDD
+1.2V REF
REFIO
ANALOG OUTPUTS
Figure 26. Dual-Supply Gain Control Circuit
In some applications, the user may elect to use an external
control amplifier to enhance the multiplying bandwidth,
distortion performance and/or settling time. External amplifiers
capable of driving a 50 pF load such as the AD817 are suitable
for this purpose. It is configured in such a way that it is in
parallel with the weaker internal reference amplifier as shown in
Figure 27. In this case, the external amplifier simply overdrives
the weaker reference control amplifier. Also, since the internal
control amplifier has a limited current output, it will sustain no
damage if overdriven.
EXTERNAL
CONTROL AMPLIFIER
AD9764
AVDD
AVDD
IOUTA
IOUTB
RLOAD
RLOAD
VREF
INPUT
REFIO
FS ADJ
RSET
50pF
CURRENT
SOURCE
ARRAY
AD9764
Figure 27. Configuring an External Reference Control
Amplifier
REV. B
Figure 28. Equivalent Analog Output Circuit
COMP1 AVDD
REFLO
+1.2V REF
IOUTA and IOUTB also have a negative and positive voltage compliance range. The negative output compliance range of –1.0 V is
set by the breakdown limits of the CMOS process. Operation
beyond this maximum limit may result in a breakdown of the
output stage and affect the reliability of the AD9764. The positive output compliance range is slightly dependent on the fullscale output current, IOUTFS. It degrades slightly from its nominal
–11–
AD9764
1.25 V for an IOUTFS = 20 mA to 1.00 V for an IOUTFS = 2 mA.
Operation beyond the positive compliance range will induce
clipping of the output signal which severely degrades the
AD9764’s linearity and distortion performance.
In summary, the AD9764 achieves the optimum distortion and
noise performance under the following conditions:
(1) Differential Operation.
(2) Positive voltage swing at IOUTA and IOUTB limited to +0.5 V.
For applications requiring the optimum dc linearity, IOUTA and/
or IOUTB should be maintained at a virtual ground via an I-V op
amp configuration. Maintaining IOUTA and/or IOUTB at a virtual
ground keeps the output impedance of the AD9764 fixed, significantly reducing its effect on linearity. However, it does not
necessarily lead to the optimum distortion performance due to
limitations of the I-V op amp. Note that the INL/DNL specifications for the AD9764 are measured in this manner using
IOUTA. In addition, these dc linearity specifications remain
virtually unaffected over the specified power supply range of
2.7 V to 5.5 V.
(3) IOUTFS set to 20 mA.
(4) Analog Supply (AVDD) set at 5.0 V.
(5) Digital Supply (DVDD) set at 3.0 V to 3.3 V with appropriate logic levels.
Note that the ac performance of the AD9764 is characterized
under the above mentioned operating conditions.
DIGITAL INPUTS
Operating the AD9764 with reduced voltage output swings at
IOUTA and IOUTB in a differential or single-ended output configuration reduces the signal dependency of its output impedance
thus enhancing distortion performance. Although the voltage
compliance range of IOUTA and I OUTB extends from –1.0 V to
+1.25 V, optimum distortion performance is achieved when the
maximum full-scale signal at IOUTA and IOUTB does not exceed
approximately 0.5 V. A properly selected transformer with a
grounded center-tap will allow the AD9764 to provide the required power and voltage levels to different loads while maintaining reduced voltage swings at IOUTA and IOUTB. DC-coupled
applications requiring a differential or single-ended output configuration should size RLOAD accordingly. Refer to Applying the
AD9764 section for examples of various output configurations.
The most significant improvement in the AD9764’s distortion
and noise performance is realized using a differential output
configuration. The common-mode error sources of both IOUTA
and IOUTB can be substantially reduced by the common-mode
rejection of a transformer or differential amplifier. These
common-mode error sources include even-order distortion
products and noise. The enhancement in distortion performance
becomes more significant as the reconstructed waveform’s
frequency content increases and/or its amplitude decreases.
This is evident in Figure 14, which compares the differential
vs. single-ended performance of the AD9764 at 50 MSPS for
0.0 and –6.0 dBFS single tone waveforms over frequency.
The distortion and noise performance of the AD9764 is also
slightly dependent on the analog and digital supply as well as the
full-scale current setting, IOUTFS. Operating the analog supply at
5.0 V ensures maximum headroom for its internal PMOS current
sources and differential switches leading to improved distortion
performance as shown in Figure 8. Although IOUTFS can be set
between 2 mA and 20 mA, selecting an IOUTFS of 20 mA will
provide the best distortion and noise performance also shown in
Figure 8. The noise performance of the AD9764 is affected by
the digital supply (DVDD), output frequency, and increases
with increasing clock rate as shown in Figure 13. Operating the
AD9764 with low voltage logic levels between 3 V and 3.3 V
will slightly reduce the amount of on-chip digital noise.
The AD9764’s digital input consists of 14 data input pins and a
clock input pin. The 14-bit parallel data inputs follow standard
positive binary coding where DB13 is the most significant bit
(MSB), and DB0 is the least significant bit (LSB). IOUTA produces a full-scale output current when all data bits are at Logic
1. IOUTB produces a complementary output with the full-scale
current split between the two outputs as a function of the input
code.
The digital interface is implemented using an edge-triggered
master slave latch. The DAC output is updated following the
rising edge of the clock as shown in Figure 1 and is designed to
support a clock rate as high as 125 MSPS. The clock can be
operated at any duty cycle that meets the specified latch pulsewidth. The setup and hold times can also be varied within the
clock cycle as long as the specified minimum times are met,
although the location of these transition edges may affect digital
feedthrough and distortion performance. Best performance is
typically achieved when the input data transitions on the falling
edge of a 50% duty cycle clock.
The digital inputs are CMOS-compatible with logic thresholds,
VTHRESHOLD, set to approximately half the digital positive supply
(DVDD) or
VTHRESHOLD = DVDD/2 (± 20%)
The internal digital circuitry of the AD9764 is capable of operating
over a digital supply range of 2.7 V to 5.5 V. As a result, the
digital inputs can also accommodate TTL levels when DVDD is
set to accommodate the maximum high level voltage of the TTL
drivers VOH(MAX). A DVDD of 3 V to 3.3 V will typically ensure
proper compatibility with most TTL logic families. Figure 29
shows the equivalent digital input circuit for the data and clock
inputs. The sleep mode input is similar with the exception that
it contains an active pull-down circuit, thus ensuring that the
AD9764 remains enabled if this input is left disconnected.
DVDD
DIGITAL
INPUT
Figure 29. Equivalent Digital Input
–12–
REV. B
AD9764
Since the AD9764 is capable of being updated up to 125 MSPS,
the quality of the clock and data input signals are important in
achieving the optimum performance. Operating the AD9764
with reduced logic swings and a corresponding digital supply
(DVDD) will result in the lowest data feedthrough and on-chip
digital noise. The drivers of the digital data interface circuitry
should be specified to meet the minimum setup and hold times
of the AD9764 as well as its required min/max input logic level
thresholds.
Digital signal paths should be kept short and run lengths
matched to avoid propagation delay mismatch. The insertion
of a low value resistor network (i.e., 20 Ω to 100 Ω) between
the AD9764 digital inputs and driver outputs may be helpful in
reducing any overshooting and ringing at the digital inputs that
contribute to data feedthrough. For longer run lengths and high
data update rates, strip line techniques with proper termination
resistors should be considered to maintain “clean” digital inputs.
30
25
IAVDD – mA
20
15
10
5
0
2
4
6
8
10
12
IOUTFS – mA
14
16
18
20
Figure 30. IAVDD vs. IOUTFS
Conversely, IDVDD is dependent on both the digital input waveform, fCLOCK, and digital supply DVDD. Figures 31 and 32
show IDVDD as a function of full-scale sine wave output ratios
(f OUT/fCLOCK) for various update rates with DVDD = 5 V and
DVDD = 3 V, respectively. Note, how IDVDD is reduced by more
than a factor of 2 when DVDD is reduced from 5 V to 3 V.
Note, that the clock input could also be driven via a sine wave,
which is centered around the digital threshold (i.e., DVDD/2)
and meets the min/max logic threshold. This will typically result
in a slight degradation in the phase noise, which becomes more
noticeable at higher sampling rates and output frequencies.
Also, at higher sampling rates, the 20% tolerance of the digital
logic threshold should be considered since it will affect the effective clock duty cycle and, subsequently, cut into the required
data setup and hold times.
18
125MSPS
16
14
100MSPS
12
IDVDD – mA
The external clock driver circuitry should provide the AD9764
with a low jitter clock input meeting the min/max logic levels
while providing fast edges. Fast clock edges will help minimize
any jitter that will manifest itself as phase noise on a reconstructed waveform. Thus, the clock input should be driven by
the fastest logic family suitable for the application.
10
8
50MSPS
6
4
SLEEP MODE OPERATION
25MSPS
2
The AD9764 has a power-down function that turns off the
output current and reduces the supply current to less than
8.5 mA over the specified supply range of 2.7 V to 5.5 V and
temperature range. This mode can be activated by applying a
logic level “1” to the SLEEP pin. This digital input also contains an active pull-down circuit that ensures the AD9764 remains enabled if this input is left disconnected. The SLEEP
input with active pull-down requires <40 µA of drive current.
5MSPS
0
0.01
0.1
RATIO – fOUT/fCLK
1
Figure 31. IDVDD vs. Ratio @ DVDD = 5 V
8
125MSPS
6
IDVDD – mA
The power-up and power-down characteristics of the AD9764
are dependent upon the value of the compensation capacitor
connected to COMP1. With a nominal value of 0.1 µF, the
AD9764 takes less than 5 µs to power down and approximately
3.25 ms to power back up. Note, the SLEEP MODE should not
be used when the external control amplifier is used as shown in
Figure 27.
100MSPS
4
50MSPS
2
25MSPS
POWER DISSIPATION
The power dissipation, PD, of the AD9764 is dependent on
several factors, including: (1) AVDD and DVDD, the power
supply voltages; (2) IOUTFS, the full-scale current output; (3)
fCLOCK, the update rate; and (4) the reconstructed digital input
waveform. The power dissipation is directly proportional to the
analog supply current, IAVDD , and the digital supply current,
IDVDD. IAVDD is directly proportional to IOUTFS, as shown in
Figure 30, and is insensitive to fCLOCK.
REV. B
0
0.01
5MSPS
0.1
RATIO – fOUT/fCLK
Figure 32. IDVDD vs. Ratio @ DVDD = 3 V
–13–
1
AD9764
APPLYING THE AD9764
DIFFERENTIAL USING AN OP AMP
OUTPUT CONFIGURATIONS
An op amp can also be used to perform a differential-to-singleended conversion as shown in Figure 34. The AD9764 is configured with two equal load resistors, RLOAD , of 25 Ω. The
differential voltage developed across IOUTA and IOUTB is converted to a single-ended signal via the differential op amp configuration. An optional capacitor can be installed across IOUTA
and IOUTB, forming a real pole in a low-pass filter. The addition
of this capacitor also enhances the op amp’s distortion performance by preventing the DAC’s high slewing output from overloading the op amp’s input.
The following sections illustrate some typical output configurations for the AD9764. Unless otherwise noted, it is assumed
that IOUTFS is set to a nominal 20 mA. For applications requiring the optimum dynamic performance, a differential output
configuration is suggested. A differential output configuration
may consist of either an RF transformer or a differential op amp
configuration. The transformer configuration provides the optimum high frequency performance and is recommended for any
application allowing for ac coupling. The differential op amp
configuration is suitable for applications requiring dc coupling, a
bipolar output, signal gain and/or level shifting.
A single-ended output is suitable for applications requiring a
unipolar voltage output. A positive unipolar output voltage will
result if IOUTA and/or IOUTB is connected to an appropriately
sized load resistor, RLOAD, referred to ACOM. This configuration may be more suitable for a single-supply system requiring a
dc coupled, ground referred output voltage. Alternatively, an
amplifier could be configured as an I-V converter, thus converting IOUTA or IOUTB into a negative unipolar voltage. This configuration provides the best dc linearity since IOUTA or IOUTB is
maintained at a virtual ground. Note, IOUTA provides slightly
better performance than IOUTB.
225V
IOUTA 22
AD8047
225V
IOUTB 21
COPT
500V
25V
25V
Figure 34. DC Differential Coupling Using an Op Amp
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-tosingle-ended signal conversion as shown in Figure 33. A
differentially coupled transformer output provides the optimum
distortion performance for output signals whose spectral content
lies within the transformer’s passband. An RF transformer such
as the Mini-Circuits T1-1T provides excellent rejection of common-mode distortion (i.e., even-order harmonics) and noise
over a wide frequency range. It also provides electrical isolation
and the ability to deliver twice the power to the load. Transformers with different impedance ratios may also be used for
impedance matching purposes. Note that the transformer
provides ac coupling only.
IOUTA 22
500V
AD9764
The common-mode rejection of this configuration is typically
determined by the resistor matching. In this circuit, the differential op amp circuit using the AD8047 is configured to provide
some additional signal gain. The op amp must operate from a
dual supply since its output is approximately ± 1.0 V. A high
speed amplifier capable of preserving the differential performance of the AD9764 while meeting other system level objectives
(i.e., cost, power) should be selected. The op amps differential
gain, its gain setting resistor values and full-scale output swing
capabilities should all be considered when optimizing this circuit.
The differential circuit shown in Figure 35 provides the necessary level-shifting required in a single supply system. In this
case, AVDD, which is the positive analog supply for both the
AD9764 and the op amp, is also used to level-shift the differential output of the AD9764 to midsupply (i.e., AVDD/2). The
AD8041 is a suitable op amp for this application.
MINI-CIRCUITS
T1-1T
500V
AD9764
AD9764
RLOAD
225V
IOUTA 22
IOUTB 21
AD8041
225V
IOUTB 21
OPTIONAL RDIFF
COPT
1kV
AVDD
Figure 33. Differential Output Using a Transformer
25V
The center tap on the primary side of the transformer must be
connected to ACOM to provide the necessary dc current path
for both IOUTA and I OUTB. The complementary voltages appearing at I OUTA and IOUTB (i.e., VOUTA and VOUTB) swing symmetrically around ACOM and should be maintained with the
specified output compliance range of the AD9764. A differential
resistor, RDIFF, may be inserted in applications in which the
output of the transformer is connected to the load, RLOAD, via a
passive reconstruction filter or cable. RDIFF is determined by the
transformer’s impedance ratio and provides the proper source
termination that results in a low VSWR. Note that approximately half the signal power will be dissipated across RDIFF.
25V
1kV
Figure 35. Single-Supply DC Differential Coupled Circuit
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 36 shows the AD9764 configured to provide a unipolar
output range of approximately 0 V to +0.5 V for a doubly terminated 50 Ω cable since the nominal full-scale current, IOUTFS, of
20 mA flows through the equivalent RLOAD of 25 Ω. In this case,
RLOAD represents the equivalent load resistance seen by IOUTA or
IOUTB. The unused output (IOUTA or IOUTB) can be connected to
ACOM directly or via a matching RLOAD. Different values of
–14–
REV. B
AD9764
IOUTFS and RLOAD can be selected as long as the positive compliance range is adhered to. One additional consideration in this
mode is the integral nonlinearity (INL) as discussed in the Analog Output section of this data sheet. For optimum INL performance, the single-ended, buffered voltage output configuration
is suggested.
AD9764
IOUTFS = 20mA
VOUTA = 0 TO +0.5V
IOUTA 22
50V
50V
system. In general, AVDD, the analog supply, should be decoupled
to ACOM, the analog common, as close to the chip as physically possible. Similarly, DVDD, the digital supply, should be
decoupled to DCOM as close as physically as possible.
For those applications requiring a single +5 V or +3 V supply
for both the analog and digital supply, a clean analog supply
may be generated using the circuit shown in Figure 38. The
circuit consists of a differential LC filter with separate power
supply and return lines. Lower noise can be attained using low
ESR type electrolytic and tantalum capacitors.
IOUTB 21
FERRITE
BEADS
25V
TTL/CMOS
LOGIC
CIRCUITS
Figure 36. 0 V to +0.5 V Unbuffered Voltage Output
AVDD
100mF
ELECT.
10-22mF
TANT.
0.1mF
CER.
ACOM
SINGLE-ENDED BUFFERED VOLTAGE OUTPUT
CONFIGURATION
Figure 37 shows a buffered single-ended output configuration in
which the op amp U1 performs an I-V conversion on the
AD9764 output current. U1 maintains IOUTA (or IOUTB) at a
virtual ground, thus minimizing the nonlinear output impedance
effect on the DAC’s INL performance as discussed in the Analog Output section. Although this single-ended configuration
typically provides the best dc linearity performance, its ac distortion performance at higher DAC update rates may be limited by
U1’s slewing capabilities. U1 provides a negative unipolar
output voltage and its full-scale output voltage is simply the
product of RFB and I OUTFS. The full-scale output should be set
within U1’s voltage output swing capabilities by scaling IOUTFS
and/or RFB . An improvement in ac distortion performance may
result with a reduced IOUTFS since the signal current U1 will be
required to sink will be subsequently reduced.
COPT
RFB
200V
AD9764
IOUTFS = 10mA
IOUTA 22
U1
VOUT = IOUTFS 3 RFB
IOUTB 21
200V
Figure 37. Unipolar Buffered Voltage Output
POWER AND GROUNDING CONSIDERATIONS
In systems seeking to simultaneously achieve high speed and
high performance, the implementation and construction of the
printed circuit board design is often as important as the circuit
design. Proper RF techniques must be used in device selection,
placement and routing and supply bypassing and grounding.
Figures 42–47 illustrate the recommended printed circuit board
ground, power and signal plane layouts that are implemented on
the AD9764 evaluation board.
Proper grounding and decoupling should be a primary objective
in any high speed, high resolution system. The AD9764 features
separate analog and digital supply and ground pins to optimize
the management of analog and digital ground currents in a
REV. B
+5V OR +3V
POWER SUPPLY
Figure 38. Differential LC Filter for Single +5 V or +3 V
Applications
Maintaining low noise on power supplies and ground is critical
to obtain optimum results from the AD9764. If properly
implemented, ground planes can perform a host of functions on
high speed circuit boards: bypassing, shielding current transport, etc. In mixed signal design, the analog and digital portions
of the board should be distinct from each other, with the analog
ground plane confined to the areas covering the analog signal
traces, and the digital ground plane confined to areas covering
the digital interconnects.
All analog ground pins of the DAC, reference and other analog
components should be tied directly to the analog ground plane.
The two ground planes should be connected by a path 1/8 to
1/4 inch wide underneath or within 1/2 inch of the DAC to
maintain optimum performance. Care should be taken to ensure
that the ground plane is uninterrupted over crucial signal paths.
On the digital side, this includes the digital input lines running
to the DAC as well as any clock signals. On the analog side, this
includes the DAC output signal, reference signal and the supply
feeders.
The use of wide runs or planes in the routing of power lines is
also recommended. This serves the dual role of providing a low
series impedance power supply to the part, as well as providing
some “free” capacitive decoupling to the appropriate ground
plane. It is essential that care be taken in the layout of signal and
power ground interconnects to avoid inducing extraneous voltage drops in the signal ground paths. It is recommended that all
connections be short, direct and as physically close to the package as possible in order to minimize the sharing of conduction
paths between different currents. When runs exceed an inch in
length, strip line techniques with proper termination resistors
should be considered. The necessity and value of this resistor
will be dependent upon the logic family used.
For a more detailed discussion of the implementation and construction of high speed, mixed signal printed circuit boards,
refer to Analog Devices’ application notes AN-280 and AN-333.
–15–
AD9764
MULTITONE PERFORMANCE CONSIDERATIONS AND
CHARACTERIZATION
The frequency domain performance of high speed DACs has
traditionally been characterized by analyzing the spectral output
of a reconstructed full-scale (i.e., 0 dBFS), single-tone sine wave
at a particular output frequency and update rate. Although this
characterization data is useful, it is often insufficient to reflect a
DAC’s performance for a reconstructed multitone or spreadspectrum waveform. In fact, evaluating a DAC’s spectral
performance using a full-scale, single tone at the highest specified
frequency (i.e., fH) of a bandlimited waveform is typically
indicative of a DAC’s “worst-case” performance for that given
waveform. In the time domain, this full-scale sine wave represents the lowest peak-to-rms ratio or crest factor (i.e., VPEAK/V
rms) that this bandlimited signal will encounter.
–10
–20
MAGNITUDE – dBm
–30
–40
–50
–60
–70
–80
–90
–100
–110
2.19 2.25 2.31 2.38 2.44 2.50 2.56 2.63 2.69 2.75 2.81
FREQUENCY – MHz
Figure 39a. Multitone Spectral Plot
1.0000
0.8000
0.6000
0.4000
VOLTS
0.2000
0.0000
–0.2000
–0.4000
–0.6000
–0.8000
–1.0000
TIME
Figure 39b. Time Domain “Snapshot” of the Multitone
Waveform
However, the inherent nature of a multitone, spread spectrum,
or QAM waveform, in which the spectral energy of the waveform is spread over a designated bandwidth, will result in a
higher peak-to-rms ratio when compared to the case of a simple
sine wave. As the reconstructed waveform’s peak-to-average
ratio increases, an increasing amount of the signal energy is
concentrated around the DAC’s midscale value. Figure 39a is
just one example of a bandlimited multitone vector (i.e., eight
tones) centered around one-half the Nyquist bandwidth (i.e.,
fCLOCK/4). This particular multitone vector, has a peak-to-rms
ratio of 13.5 dB compared to a sine waves peak-to-rms ratio of
3 dB. A “snapshot” of this reconstructed multitone vector in the
time domain as shown in Figure 39b reveals the higher signal
content around the midscale value. As a result, a DAC’s
“small-scale” dynamic and static linearity becomes increasingly critical in obtaining low intermodulation distortion and
maintaining sufficient carrier-to-noise ratios for a given modulation scheme.
A DAC’s small-scale linearity performance is also an important
consideration in applications where additive dynamic range is
required for gain control purposes or “predistortion” signal
conditioning. For instance, a DAC with sufficient dynamic
range can be used to provide additional gain control of its
reconstructed signal. In fact, the gain can be controlled in
6 dB increments by simply performing a shift left or right on the
DAC’s digital input word. Other applications may intentionally
predistort a DAC’s digital input signal to compensate for
nonlinearities associated with the subsequent analog components in the signal chain. For example, the signal compression
associated with a power amplifier can be compensated for by
predistorting the DAC’s digital input with the inverse nonlinear
transfer function of the power amplifier. In either case, the
DAC’s performance at reduced signal levels should be carefully
evaluated.
A full-scale single tone will induce all of the dynamic and static
nonlinearities present in a DAC that contribute to its distortion
and hence SFDR performance. Referring to Figure 3, as the
frequency of this reconstructed full-scale, single-tone waveform
increases, the dynamic nonlinearities of any DAC (i.e., AD9764)
tend to dominate thus contributing to the rolloff in its SFDR
performance. However, unlike most DACs, which employ an R-2R
ladder for the lower bit current segmentation, the AD9764 (as
well as other TxDAC members) exhibits an improvement in
distortion performance as the amplitude of a single tone is reduced from its full-scale level. This improvement in distortion
performance at reduced signal levels is evident if one compares
the SFDR performance vs. frequency at different amplitudes
(i.e., 0 dBFS, –6 dBFS and –12 dBFS) and sample rates as
shown in Figures 4 through 7. Maintaining decent “small-scale”
linearity across the full span of a DAC transfer function is also
critical in maintaining excellent multitone performance.
Although characterizing a DAC’s multitone performance tends
to be application-specific, much insight into the potential performance of a DAC can also be gained by evaluating the DAC’s
swept power (i.e., amplitude) performance for single, dual and
multitone test vectors at different clock rates and carrier frequencies. The DAC is evaluated at different clock rates when reconstructing a specific waveform whose amplitude is decreased in
3 dB increments from full-scale (i.e., 0 dBFS). For each specific
waveform, a graph showing the SFDR (over Nyquist) performance vs. amplitude can be generated at the different tested
clock rates as shown in Figures 9–11. Note that the carrier(s)to-clock ratio remains constant in each figure. In each case, an
improvement in SFDR performance is seen as the amplitude is
reduced from 0 dBFS to approximately –9.0 dBFS.
A multitone test vector may consist of several equal amplitude,
spaced carriers each representative of a channel within a defined
bandwidth as shown in Figure 39a. In many cases, one or more
tones are removed so the intermodulation distortion performance
–16–
REV. B
AD9764
of the DAC can be evaluated. Nonlinearities associated with the
DAC will create spurious tones of which some may fall back into
the “empty” channel thus limiting a channel’s carrier-to-noise
ratio. Other spurious components falling outside the band of
interest may also be important, depending on the system’s spectral
mask and filtering requirements.
AD9764 EVALUATION BOARD
General Description
This particular test vector was centered around one-half the
Nyquist bandwidth (i.e., fCLOCK/4) with a passband of fCLOCK/16.
Centering the tones at a much lower region (i.e., fCLOCK/10)
would lead to an improvement in performance while centering
the tones at a higher region (i.e., fCLOCK/2.5) would result in a
degradation in performance. Figure 40a shows the SFDR vs.
amplitude at different sample rates up to the Nyquist frequency
while Figure 40b shows the SFDR vs. amplitude within the
passband of the test vector. In assessing a DAC’s multitone
performance, it is also recommended that several units be tested
under exactly the same conditions to determine any performance
variability.
80
The AD9764-EB is an evaluation board for the AD9764 14-bit
DAC converter. Careful attention to layout and circuit design,
combined with a prototyping area, allows the user to easily and
effectively evaluate the AD9764 in any application where high
resolution, high speed conversion is required.
This board allows the user the flexibility to operate the AD9764
in various configurations. Possible output configurations include
transformer coupled, resistor terminated, inverting/noninverting
and differential amplifier outputs. The digital inputs are designed
to be driven directly from various word generators with the
onboard option to add a resistor network for proper load termination. Provisions are also made to operate the AD9764 with
either the internal or external reference or to exercise the powerdown feature.
Refer to the application note AN-420, Using the AD9760/AD9764/
AD9764-EB Evaluation Board for a thorough description and
operating instructions for the AD9764 evaluation board.
75
70
10 MSPS
SFDR – dBc
65
60
20 MSPS
55
50
50 MSPS
45
40
100 MSPS
35
30
–20
–15
–10
AOUT – dBFS
–5
0
Figure 40a. Multitone SFDR vs. AOUT (Up to Nyquist)
80
10 MSPS
20 MSPS
75
SFDR – dBc
70
65
60
50 MSPS
55
100 MSPS
50
45
40
–20
–15
–10
AOUT – dBFS
–5
0
Figure 40b. Multitone SFDR vs. AOUT (Within Multitone
Passband)
REV. B
–17–
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
–18–
A
J4
A
J3
P1
A
R38
49.9V
A
OUT2
R20
49.9V
OUT1
R2
C13
22pF
R14
0
C12
22pF
A
6
5
4
1
2 3 4 5 6 7 8 9 10
10 9 8 7 6 5 4 3 2
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
1
R1
R6
1
T1
3
C4
10mF
TP4
B3
A
J7
JP6B
C20
0
JP6A
A
C30
C31
C32
C33
C34
C35
C36
C19
C1
C2
C25
C26
C27
C28
C29
A
AGND
R13
OPEN
A
R12
OPEN
DVDD
1
2 3 4 5 6 7 8 9 10
AVDD
10 9 8 7 6 5 4 3 2
1
R5
DVDD
TP2
TP3
C3
10mF
B2
DGND
B1
DVDD
16
15
14
13
12
11
10
9
A
B
JP7B
1
2
3
4
5
6
7
R9
1kV
16
15
14
13
12
11
10
16 PINDIP
RES PK
1
2
3
4
5
6
7
8
JP9
A
C6
10mF
TP7
B6
R4
B
A
A
B
R10
1kV
A
R35
1kV
A
B
JP8
R18
1kV
2
3
U4
7
AVCC
R8
AVEE
4
A
C23
0.1mF
R36
1kV
6
A
C21
0.1mF
C24
1mF
R37
49.9V
C22
1mF
A
J6
DVDD
1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
J1
2 3 4 5 6 7 8 9 10
EXTCLK
10 9 8 7 6 5 4 3 2
1
R7
DVDD
A
AD8047
1
2 3 4 5 6 7 8 9 10
A
AVCC
10 9 8 7 6 5 4 3 2
1
R3
C5
10mF
TP6
B5
JP7A
AVEE
16 PINDIP
RES PK
TP5
TP18
TP19
B4
A
C18
0.1mF
TP12
A
28
27
26
25
24
23
22
21
20
19
18
17
16
15
2
CLK
JP1
A
C16
1mF
TP11
U7
A
A
4
GND
R44
50V
VOUT
REF43
A
3
2
1
AVDD
JP2
VIN
A
C7
1mF
AVDD
EXTREFIN
J5
2
PDIN
J2
A
3
B
AVCC
R17
49.9V
CLOCK
DVDD
DCOM
NC
AVDD
COMP2
IOUTA
IOUTB
ACOM
COMP1
FS ADJ
REFIO
REFLO
SLEEP
CT1
DB13
DB12
DB11
DB10
DB9
DB8
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
AD9764
U1
1
R15
49.9V
TP1
C14
1mF
6
A
R45
1kV
R43
5kV
R42
1kV
A
A
JP4
A
R46
1kV
2
3
7
A
3
4
A
AD8047
U6
JP3
1
AVEE
A
C15
0.1mF
2
JP5
6
C17
0.1mF
AVCC
C10
0.1mF
TP9
OUT 2
TP8
OUT 1
C9
0.1mF
AVDD
CW
TP14
R16
2kV
TP10
AVDD
C11
0.1mF
C8
0.1mF
1
2
3
A
B
TP13
AD9764
Figure 41. Evaluation Board Schematic
REV. B
AD9764
Figure 42. Silkscreen Layer—Top
Figure 43. Component Side PCB Layout (Layer 1)
REV. B
–19–
AD9764
Figure 44. Ground Plane PCB Layout (Layer 2)
Figure 45. Power Plane PCB Layout (Layer 3)
–20–
REV. B
AD9764
Figure 46. Solder Side PCB Layout (Layer 4)
Figure 47. Silkscreen Layer—Bottom
REV. B
–21–
AD9764
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
C2467b–1–10/99
28-Lead, 300 Mil SOIC
(R-28)
0.7125 (18.10)
0.6969 (17.70)
28
15
0.2992 (7.60)
0.2914 (7.40)
1
0.4193 (10.65)
0.3937 (10.00)
14
PIN 1
0.1043 (2.65)
0.0926 (2.35)
0.0118 (0.30)
0.0040 (0.10)
0.0500
(1.27)
BSC
0.0291 (0.74)
3 458
0.0098 (0.25)
88
08
0.0192 (0.49) SEATING
0.0125 (0.32)
PLANE
0.0138 (0.35)
0.0091 (0.23)
0.0500 (1.27)
0.0157 (0.40)
28-Lead TSSOP
(RU-28)
0.386 (9.80)
0.378 (9.60)
28
15
0.177 (4.50)
0.169 (4.30)
0.256 (6.50)
0.246 (6.25)
1
14
PIN 1
0.006 (0.15)
0.002 (0.05)
0.0256 (0.65)
BSC
0.0118 (0.30)
0.0075 (0.19)
0.0079 (0.20)
0.0035 (0.090)
88
08
0.028 (0.70)
0.020 (0.50)
PRINTED IN U.S.A.
SEATING
PLANE
0.0433 (1.10)
MAX
–22–
REV. B