REV. 0
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.
a
AD9750
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
10-Bit, 125 MSPS High Performance
TxDAC
®
D/A Converter
FUNCTIONAL BLOCK DIAGRAM
150pF
+1.20V REF AVDD ACOM
REFLO
ICOMP
CURRENT
SOURCE
ARRAY
+5V
SEGMENTED
SWITCHES LSB
SWITCH
REFIO
FS ADJ
DVDD
DCOM
CLOCK
+5V
R
SET
0.1mF
CLOCK
IOUTA
IOUTB
0.1mF
LATCHES
AD9750
SLEEP
DIGITAL DATA INPUTS (DB9–DB0)
FEATURES
High Performance Member of Pin-Compatible
TxDAC Product Family
125 MSPS Update Rate
10-Bit Resolution
Excellent Spurious Free Dynamic Range Performance
SFDR to Nyquist @ 5 MHz Output: 76 dBc
Differential Current Outputs: 2 mA to 20 mA
Power Dissipation: 190 mW @ 5 V
Power-Down Mode: 20 mW @ 5 V
On-Chip 1.20 V Reference
CMOS-Compatible +2.7 V to +5.5 V Digital Interface
Packages: 28-Lead SOIC and TSSOP
Edge-Triggered Latches
APPLICATIONS
Wideband Communication Transmit Channel:
Direct IF
Basestations
Wireless Local Loop
Digital Radio Link
Direct Digital Synthesis (DDS)
Instrumentation
The AD9750 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.
The digital inputs support +2.7 V and +5 V CMOS logic families.
TxDAC is a registered trademark of Analog Devices, Inc.
*Protected by U.S. Patents Numbers 5450084, 5568145, 5689257, 5612697 and
5703519.
The AD9750 is a current-output DAC with a nominal full-scale
output current of 20 mA and > 100 kΩ output impedance.
Differential current outputs are provided to support single-
ended 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 complemen-
tary, single-ended voltage outputs or fed directly into a trans-
former. The output voltage compliance range is 1.25 V.
The on-chip reference and control amplifier are configured for
maximum accuracy and flexibility. The AD9750 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 AD9750 full-scale current
to be adjusted over a 2 mA to 20 mA range while maintaining
excellent dynamic performance. Thus, the AD9750 may oper-
ate at reduced power levels or be adjusted over a 20 dB range to
provide additional gain ranging capabilities.
The AD9750 is available in 28-lead SOIC and TSSOP packages.
It is specified for operation over the industrial temperature range.
PRODUCT HIGHLIGHTS
1. The AD9750 is a member of the wideband TxDAC
high per-
formance product family that provides an upward or downward
component selection path based on resolution (8 to 14 bits),
performance and cost. The entire family of TxDACs is avail-
able in industry standard pinouts.
2. Manufactured on a CMOS process, the AD9750 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 interface to
+2.7 V to +5 V CMOS logic families. The AD9750 can
support update rates up to 125 MSPS.
4. A flexible single-supply operating range of +4.5 V to +5.5 V,
and a wide full-scale current adjustment span of 2 mA to
20 mA, allows the AD9750 to operate at reduced power levels.
5. The current output(s) of the AD9750 can be easily config-
ured for various single-ended or differential circuit topologies.
PRODUCT DESCRIPTION
The AD9750 is a 10-bit resolution, wideband, second generation
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 commu-
nication systems. All of the devices share the same interface
options, small outline package and pinout, thus providing an up-
ward or downward component selection path based on perfor-
mance, resolution and cost. The AD9750 offers exceptional ac and
dc performance while supporting update rates up to 125 MSPS.
The AD9750’s flexible single-supply operating range of 4.5 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 65 mW, without a significant degradation in
performance, by lowering the full-scale current output. Also, a
power-down mode reduces the standby power dissipation to
apprixmatley 20 mW.
AD9750–SPECIFICATIONS
–2– REV. 0
DC SPECIFICATIONS
Parameter Min Typ Max Units
RESOLUTION 10 Bits
DC ACCURACY
1
Integral Linearity Error (INL) –1.0 ±0.1 +1.0 LSB
Differential Nonlinearity (DNL) –0.5 ±0.1 +0.5 LSB
ANALOG OUTPUT
Offset Error –0.02 +0.02 % of FSR
Gain Error
(Without Internal Reference) –2 ±0.5 +2 % of FSR
Gain Error
(With Internal Reference) –5 ±1.5 +5 % of FSR
Full-Scale Output Current
2
2.0 20.0 mA
Output Compliance Range –1.0 1.25 V
Output Resistance 100 kΩ
Output Capacitance 5 pF
REFERENCE OUTPUT
Reference Voltage 1.14 1.20 1.26 V
Reference Output Current
3
100 nA
REFERENCE INPUT
Input Compliance Range 0.1 1.25 V
Reference Input Resistance 1 MΩ
Small Signal Bandwidth 0.5 MHz
TEMPERATURE COEFFICIENTS
Offset Drift 0 ppm of FSR/°C
Gain Drift
(Without Internal Reference) ±50 ppm of FSR/°C
Gain Drift
(With Internal Reference) ±100 ppm of FSR/°C
Reference Voltage Drift ±50 ppm/°C
POWER SUPPLY
Supply Voltages
AVDD 4.5 5.0 5.5 V
DVDD 2.7 5.0 5.5 V
Analog Supply Current (I
AVDD
)
4
33 39 mA
Digital Supply Current (I
DVDD
)
5
5.0 7 mA
Supply Current Sleep Mode (I
AVDD
)
6
4.0 8 mA
Power Dissipation
5
(5 V, I
OUTFS
= 20 mA) 190 230 mW
Power Supply Rejection Ratio
7
—AVDD –0.4 +0.4 % of FSR/V
Power Supply Rejection Ratio
7
—DVDD –0.025 +0.025 % of FSR/V
OPERATING RANGE –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
Requires +5 V supply.
5
Measured at f
CLOCK
= 50 MSPS and I
OUT
= static full scale (20 mA).
6
Logic level for SLEEP pin must be referenced to AVDD. Min V
IH
= 3.5 V.
7
±5% Power supply variation.
Specifications subject to change without notice.
(TMIN to TMAX, AVDD = +5 V, DVDD = +5 V, IOUTFS = 20 mA, unless otherwise noted)
–3–
REV. 0
AD9750
DYNAMIC SPECIFICATIONS
Parameter Min Typ Max Units
DYNAMIC PERFORMANCE
Maximum Output Update Rate (f
CLOCK
) 125 MSPS
Output Settling Time (t
ST
) (to 0.1%)
1
35 ns
Output Propagation Delay (t
PD
)1ns
Glitch Impulse 5 pV-s
Output Rise Time (10% to 90%)
1
2.5 ns
Output Fall Time (10% to 90%)
1
2.5 ns
Output Noise (I
OUTFS
= 20 mA) 50 pA/√Hz
Output Noise (I
OUTFS
= 2 mA) 30 pA/√Hz
AC LINEARITY
Spurious-Free Dynamic Range to Nyquist
f
CLOCK
= 50 MSPS; f
OUT
= 1.00 MHz
0 dBFS Output
T
A
= +25°C 71 82 dBc
f
CLOCK
= 50 MSPS; f
OUT
= 2.51 MHz 79 dBc
f
CLOCK
= 50 MSPS; f
OUT
= 5.02 MHz 76 dBc
f
CLOCK
= 50 MSPS; f
OUT
= 20.2 MHz 60 dBc
f
CLOCK
= 100 MSPS; f
OUT
= 2.51 MHz 78 dBc
f
CLOCK
= 100 MSPS; f
OUT
= 5.04 MHz 77 dBc
f
CLOCK
= 100 MSPS; f
OUT
= 20.2 MHz 69 dBc
f
CLOCK
= 100 MSPS; f
OUT
= 40.4 MHz 63 dBc
Spurious-Free Dynamic Range within a Window
f
CLOCK
= 50 MSPS; f
OUT
= 1.00 MHz 80 87 dBc
f
CLOCK
= 50 MSPS; f
OUT
= 5.02 MHz; 2 MHz Span 86 dBc
f
CLOCK
= 100 MSPS; f
OUT
= 5.04 MHz; 4 MHz Span 86 dBc
Total Harmonic Distortion
f
CLOCK
= 50 MSPS; f
OUT
= 1.00 MHz
T
A
= +25°C –80 dBc
f
CLOCK
= 50 MHz; f
OUT
= 2.00 MHz –76 dBc
f
CLOCK
= 100 MHz; f
OUT
= 2.00 MHz –76 dBc
NOTES
1
Measured single ended into 50 Ω load.
Specifications subject to change without notice.
(TMIN to TMAX, AVDD = +5 V, DVDD = +5 V, IOUTFS = 20 mA, Differential Transformer Coupled Output,
50 ⍀ Doubly Terminated, unless otherwise noted)
AD9750
–4– REV. 0
DIGITAL SPECIFICATIONS
Parameter Min Typ Max Units
DIGITAL INPUTS
Logic “1” Voltage @ DVDD = +5 V
1
3.5 5 V
Logic “1” Voltage @ DVDD = +3 V 2.1 3 V
Logic “0” Voltage @ DVDD = +5 V
1
0 1.3 V
Logic “0” Voltage @ DVDD = +3 V 0 0.9 V
Logic “1” Current –10 +10 µA
Logic “0” Current –10 +10 µA
Input Capacitance 5 pF
Input Setup Time (t
S
) 2.0 ns
Input Hold Time (t
H
) 1.5 ns
Latch Pulsewidth (t
LPW
) 3.5 ns
NOTES
1
When DVDD = +5 V, and Logic 1 voltage ≈3.5 V and Logic 0 voltage ≈1.3 V, IVDD can increase by up to 10 mA depending on f
CLOCK
.
Specifications subject to change without notice.
0.1%
0.1%
t
S
t
H
t
LPW
t
PD
t
ST
DB0–DB11
CLOCK
IOUTA
OR
IOUTB
Figure 1. Timing Diagram
ABSOLUTE MAXIMUM RATINGS*
With
Parameter Respect to Min Max Units
AVDD ACOM –0.3 +6.5 V
DVDD DCOM –0.3 +6.5 V
ACOM DCOM –0.3 +0.3 V
AVDD DVDD –6.5 +6.5 V
CLOCK, SLEEP DCOM –0.3 DVDD + 0.3 V
Digital Inputs DCOM –0.3 DVDD + 0.3 V
IOUTA, IOUTB ACOM –1.0 AVDD + 0.3 V
ICOMP ACOM –0.3 AVDD + 0.3 V
REFIO, FSADJ ACOM –0.3 AVDD + 0.3 V
REFLO ACOM –0.3 +0.3 V
Junction Temperature +150 °C
Storage Temperature –65 +150 °C
Lead Temperature
(10 sec) +300 °C
*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 indicated in the operational
sections of this specification is not implied. Exposure to absolute maximum
ratings for extended periods may effect device reliability.
ORDERING GUIDE
Temperature Package Package
Model Ranges Descriptions Options*
AD9750AR –40°C to +85°C 28-Lead 300 Mil SOIC R-28
AD9750ARU –40°C to +85°C 28-Lead TSSOP RU-28
AD9750-EB Evaluation Board
*R = Small Outline IC; RU = Thin Shrink Small Outline Package.
THERMAL CHARACTERISTICS
Thermal Resistance
28-Lead 300 Mil SOIC
θ
JA
= 71.4°C/W
θ
JC
= 23°C/W
28-Lead TSSOP
θ
JA
= 97.9°C/W
θ
JC
= 14.0°C/W
(TMIN to TMAX, AVDD = +5 V, DVDD = +5 V, IOUTFS = 20 mA unless otherwise noted)
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 AD9750 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.
WARNING!
ESD SENSITIVE DEVICE
AD9750
–5–
REV. 0
PIN CONFIGURATION
14
13
12
11
17
16
15
20
19
18
10
9
8
1
2
3
4
7
6
5
TOP VIEW
(Not to Scale)
28
27
26
25
24
23
22
21
AD9750
NC = NO CONNECT
(MSB) DB9
DB8
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
NC
NC
NC
NC
CLOCK
DVDD
DCOM
NC
AVDD
ICOMP
IOUTA
IOUTB
ACOM
NC
FS ADJ
REFIO
REFLO
SLEEP
PIN FUNCTION DESCRIPTIONS
Pin No. Name Description
1 DB9 Most Significant Data Bit (MSB).
2–9 DB8–DB1 Data Bits 1–8.
10 DB0 Least Significant Data Bit (LSB).
11–14, 19, 25 NC No Internal Connection.
15 SLEEP Power-Down Control Input. Active High. Contains active pull-down circuit, thus may be left unterminated
if not used.
16 REFLO Reference Ground when Internal 1.2 V Reference Used. Connect to AVDD to disable internal
reference.
17 REFIO 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.
18 FS ADJ Full-Scale Current Output Adjust.
20 ACOM Analog Common.
21 IOUTB Complementary DAC Current Output. Full-scale current when all data bits are 0s.
22 IOUTA DAC Current Output. Full-scale current when all data bits are 1s.
23 ICOMP Internal Bias Node for Switch Driver Circuitry. Decouple to ACOM with 0.1 µF capacitor.
24 AVDD Analog Supply Voltage (+4.5 V to +5.5 V).
26 DCOM Digital Common.
27 DVDD Digital Supply Voltage (+2.7 V to +5.5 V).
28 CLOCK Clock Input. Data latched on positive edge of clock.
AD9750
–6– REV. 0
DEFINITIONS OF SPECIFICATIONS
Linearity Error (Also Called Integral Nonlinearity or INL)
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.
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.
Monotonicity
A D/A converter is monotonic if the output either increases or
remains constant as the digital input increases.
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.
Gain Error
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.
Output Compliance Range
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.
Temperature Drift
Temperature drift is specified as the maximum change from the
ambient (+25°C) value to the value at either T
MIN
or T
MAX
. 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.
Power Supply Rejection
The maximum change in the full-scale output as the supplies
are varied from nominal to minimum and maximum specified
voltages.
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.
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.
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.
Total Harmonic Distortion
THD is the ratio of the rms sum of the first six harmonic
components to the rms value of the measured input signal. It is
expressed as a percentage or in decibels (dB).
Multitone Power Ratio
The spurious-free dynamic range for an output containing mul-
tiple 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.
+1.20V REF
AVDD ACOM
REFLO
ICOMP
PMOS
CURRENT SOURCE
ARRAY
+5V
SEGMENTED SWITCHES
FOR DB9–DB1 LSB
SWITCH
REFIO
FS ADJ
DVDD
DCOM
CLOCK
+5V
R
SET
2kV
0.1mF
DVDD
DCOM
IOUTA
IOUTB
0.1mF
AD9750
SLEEP
50V
RETIMED
CLOCK
OUTPUT*
LATCHES
DIGITAL
DATA
TEKTRONIX
AWG-2021
W/OPTION 4
LECROY 9210
PULSE GENERATOR
CLOCK
OUTPUT
50V20pF
50V20pF
100V
TO HP3589A
SPECTRUM/
NETWORK
ANALYZER
50V INPUT
MINI-CIRCUITS
T1-1T
* AWG2021 CLOCK RETIMED
SUCH THAT DIGITAL DATA
TRANSITIONS ON FALLING EDGE
OF 50% DUTY CYCLE CLOCK.
150pF
Figure 2. Basic AC Characterization Test Set-Up
AD9750
–7–
REV. 0
Typical AC Characterization Curves @ +5 V Supplies
(AVDD = +5 V, DVDD = +5 V, IOUTFS = 20 mA, 50 ⍀ Doubly Terminated Load, Differential Output, TA = +25ⴗC, SFDR up to Nyquist, unless otherwise noted)
f
OUT
– MHz
SFDR – dBc
90
80
40 110
70
60
50
25MSPS
100MSPS
65MSPS
125MSPS
Figure 3. SFDR vs. f
OUT
@ 0 dBFS
f
OUT
– MHz
SFDR – dBc
90
80
40010 20 30 40 50
70
60
50
–12dBF
S
–6dBF
S
0dBF
S
Figure 6. SFDR vs. f
OUT
@ 100 MSPS
A
OUT
– dBF
S
SFDR – dBc
90
80
–25 –20 –15 –10 –5 0
70
60
50
455kHz/5MSPS
5.91/65MSPS
2.27MHz/25MHz
11.37MHz/125MSPS
Figure 9. Single-Tone SFDR vs. A
OUT
@ f
OUT
= f
CLOCK
/11
f
OUT
– MHz
SFDR – dBc
90
80
40021246810
70
60
50
–6dBF
S
–12dBF
S
0dBF
S
Figure 4. SFDR vs. f
OUT
@ 25 MSPS
f
OUT
– MHz
SFDR – dBc
90
80
40010 60
20 30 40 50
70
60
50
–12dBF
S
–6dBF
S
0dBF
S
Figure 7. SFDR vs. f
OUT
@125 MSPS
A
OUT
– dBc
SFDR – dBc
90
80
–25 –20 –15 –10 –5 0
70
60
50
1MHz/5MHz
13MHz/65MHz
25MHz/125MHz
5MHz/25MHz
Figure 10. Single-Tone SFDR vs.
A
OUT
@ f
OUT
= f
CLOCK
/5
f
OUT
– MHz
SFDR – dBc
90
80
400530
10 15 20 25
70
60
50
–12dBF
S
–6dBF
S
0dBF
S
Figure 5. SFDR vs. f
OUT
@ 65 MSPS
f
OUT
– MHz
SFDR – dBc
90
80
400246810
70
60
50
20mAF
S
10mAF
S
5mAF
S
12
Figure 8. SFDR vs. f
OUT
and I
OUTFS
@
25 MSPS and 0 dBFS
f
CLOCK
– MSPS
SNR – dB
70
66
50010 60
20 30 40 50
62
58
54
I
OUTFS
= 20mA
I
OUTFS
= 5mA I
OUTFS
= 10mA
Figure 11. SNR vs. f
CLOCK
and I
OUTFS
@ f
OUT
= 2 MHz and 0 dBFS
AD9750
–8– REV. 0
CODE
0.2
0.15
–0.101000
800
ERROR – LSB
0.1
0.05
0
–0.05
200 400 600
Figure 12. Typical INL
f
OUT
– MHz
AMPLITUDE – dBm
0
–10
–40
620
8101214
–20
–30
16 184
–50
–60
–70
–80
–90
–100 22 24
f
CLOCK
= 125MSPS
f
OUT1
= 13.5MHz
f
OUT2
= 14.5MHz
AMPLITUDE = 0dBF
S
SDFR = 75dBc
Figure 15. Two-Tone SFDR
CODE
0.08
0.04
–0.1201000
800
ERROR – LSB
0
–0.04
–0.08
200 400 600
Figure 13. Typical DNL
f
OUT
– MHz
–10
–20
–90
030
20
AMPLITUDE – dBm
–30
–40
–50
51015
–60
–70
–80
25
f
CLOCK
= 65MSPS
f
OUT1
= 6.25MHz
f
OUT2
= 6.75MHz
f
OUT3
= 7.25MHz
f
OUT4
= 7.75MHz
AMPLITUDE = 0dBF
S
SFDR = 70dBc
–100
Figure 16. Four-Tone SFDR
TEMPERATURE – 8C
SFDR – dBc
90
80
50 –40 100
–20 0 20 40
70
60
60 80–60
f
OUT
= 2.5MHz
f
OUT
= 10MHz
f
OUT
= 40MHz
Figure 14. SFDR vs. Temperature
@ 125 MSPS, 0 dBFS
AD9750
–9–
REV. 0
FUNCTIONAL DESCRIPTION
Figure 17 shows a simplified block diagram of the AD9750.
The AD9750 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 LSB is a binary weighted frac-
tions 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.
The analog and digital sections of the AD9750 have separate
power supply inputs (i.e., AVDD and DVDD). The digital
section, which is capable of operating up to a 125 MSPS clock
rate and over a +2.7 V to +5.5 V operating range, consists of
edge-triggered latches and segment decoding logic circuitry.
The analog section, which can operate over a +4.5 V to +5.5 V
range, includes the PMOS current sources, the associated differ-
ential switches, a 1.20 V bandgap voltage reference and a refer-
ence control amplifier.
The full-scale output current is regulated by the reference con-
trol amplifier and can be set from 2 mA to 20 mA via an exter-
nal resistor, R
SET
. The external resistor, in combination with
both the reference control amplifier and voltage reference V
REFIO
,
sets the reference current I
REF
, which is mirrored over to the
segmented current sources with the proper scaling factor. The
full-scale current, I
OUTFS
, is thirty-two times the value of I
REF
.
DAC TRANSFER FUNCTION
The AD9750 provides complementary current outputs, IOUTA
and IOUTB. IOUTA will provide a near full-scale current output,
I
OUTFS
, when all bits are high (i.e., DAC CODE = 1023) 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 I
OUTFS
and can be expressed as:
IOUTA = (DAC CODE/1024) × I
OUTFS
(1)
IOUTB = (1023 – DAC CODE)/1024 × I
OUTFS
(2)
where DAC CODE = 0 to 1023 (i.e., Decimal Representation).
As mentioned previously, I
OUTFS
is a function of the reference
current I
REF
, which is nominally set by a reference voltage
V
REFIO
and external resistor R
SET
. It can be expressed as:
I
OUTFS
= 32 × I
REF
(3)
where I
REF
= V
REFIO
/R
SET
(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, R
LOAD
, which are tied to analog common, ACOM. Note,
R
LOAD
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 :
V
OUTA
= IOUTA × R
LOAD
(5)
V
OUTB
= IOUTB × R
LOAD
(6)
Note the full-scale value of V
OUTA
and V
OUTB
should not exceed
the specified output compliance range to maintain specified
distortion and linearity performance.
The differential voltage, V
DIFF
, appearing across IOUTA and
IOUTB is:
V
DIFF
= (IOUTA – IOUTB) × R
LOAD
(7)
Substituting the values of IOUTA, IOUTB, and I
REF
; V
DIFF
can
be expressed as:
V
DIFF
= {(2 DAC CODE – 1023)/1024} ×
(32 R
LOAD
/R
SET
) × V
REFIO
(8)
These last two equations highlight some of the advantages of
operating the AD9750 differentially. First, the differential op-
eration will help cancel common-mode error sources associated
with IOUTA and IOUTB such as noise, distortion and dc off-
sets. Second, the differential code dependent current and subse-
quent voltage, V
DIFF
, is twice the value of the single-ended
voltage output (i.e., V
OUTA
or V
OUTB
), thus providing twice the
signal power to the load.
Note, the gain drift temperature performance for a single-ended
(V
OUTA
and V
OUTB
) or differential output (V
DIFF
) of the AD9750
can be enhanced by selecting temperature tracking resistors for
R
LOAD
and R
SET
due to their ratiometric relationship as shown
in Equation 8.
DIGITAL DATA INPUTS (DB9–DB0)
150pF
+1.20V REF
AVDD ACOM
REFLO
ICOMP
PMOS
CURRENT SOURCE
ARRAY
+5V
SEGMENTED SWITCHES
FOR DB9–DB1 LSB
SWITCH
REFIO
FS ADJ
DVDD
DCOM
CLOCK
+5V
R
SET
2kV
0.1mF
IOUTA
IOUTB
0.1mF
AD9750
SLEEP LATCHES
I
REF
V
REFIO
CLOCK
I
OUTB
I
OUTA
R
LOAD
50V
V
OUTB
V
OUTA
R
LOAD
50V
V
DIFF
= V
OUTA
– V
OUTB
Figure 17. Functional Block Diagram
AD9750
–10– REV. 0
REFERENCE OPERATION
The AD9750 contains an internal 1.20 V bandgap reference
that can easily be disabled and overridden by an external refer-
ence. REFIO serves as either an input or output depending on
whether the internal or an external reference is selected. If
REFLO is tied to ACOM, as shown in Figure 18, 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.
150pF
+1.2V REF
AVDD
REFLO
CURRENT
SOURCE
ARRAY
+5V
REFIO
FS ADJ
2kV
0.1mF
AD9750
ADDITIONAL
LOAD
OPTIONAL
EXTERNAL
REF BUFFER
Figure 18. Internal Reference Configuration
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 19. 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 im-
pedance (i.e., 1 MΩ) of REFIO minimizes any loading of the
external reference.
150pF
+1.2V REF
AVDD
REFLO
CURRENT
SOURCE
ARRAY
AVDD
REFIO
FS ADJ
RSET
AD9750
EXTERNAL
REF
IREF =
VREFIO/RSET
AVDD
REFERENCE
CONTROL
AMPLIFIER
VREFIO
Figure 19. External Reference Configuration
REFERENCE CONTROL AMPLIFIER
The AD9750 also contains an internal control amplifier that is
used to regulate the DAC’s full-scale output current, I
OUTFS
.
The control amplifier is configured as a V-I converter as shown
in Figure 19, such that its current output, I
REF
, is determined by
the ratio of the V
REFIO
and an external resistor, R
SET
, as stated
in Equation 4. I
REF
is copied over to the segmented current
sources with the proper scaling factor to set I
OUTFS
as stated in
Equation 3.
The control amplifier allows a wide (10:1) adjustment span of
I
OUTFS
over a 2 mA to 20 mA range by setting IREF between
62.5 µA and 625 µA. The wide adjustment span of I
OUTFS
provides several application benefits. The first benefit relates
directly to the power dissipation of the AD9750, which is
proportional to I
OUTFS
(refer to the Power Dissipation section).
The second benefit relates to the 20 dB adjustment, which is
useful for system gain control purposes.
The small signal bandwidth of the reference control amplifier is
approximately 0.5 MHz. The output of the control amplifier is
internally compensated via a 150 pF capacitor that limits the
control amplifier small-signal bandwidth and reduces its
output impedance. 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 re-
sponse can be approximated. In this case, the time constant can
be approximated to be 320 ns.
There are two methods in which I
REF
can be varied for a fixed
R
SET
. 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 I
REF
to be varied for a fixed R
SET
. 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 20 using the AD7524 and an external 1.2 V reference,
the AD1580.
The second method may be used in a dual-supply system in
which the common-mode voltage of REFIO is fixed and I
REF
is
varied by an external voltage, V
GC
, applied to R
SET
via an ampli-
fier. An example of this method is shown in Figure 21, in which
the internal reference is used to set the common-mode voltage
of the control amplifier to 1.20 V. The external voltage, V
GC
, is
referenced to ACOM and should not exceed 1.2 V. The value of
R
SET
is such that I
REFMAX
and I
REFMIN
do not exceed 62.5 µA
1.2V
150pF
+1.2V REF
AVDD
REFLO
CURRENT
SOURCE
ARRAY
AVDD
REFIO
FS ADJ
R
SET
AD9750
I
REF
=
V
REF
/R
SET
AVDD
V
REF
V
DD
R
FB
OUT1
OUT2
AGND
DB7–DB0
AD7524
AD1580
0.1V TO 1.2V
Figure 20. Single-Supply Gain Control Circuit
AD9750
–11–
REV. 0
and 625 µA, respectively. The associated equations in Figure 21
can be used to determine the value of R
SET
.
150pF
+1.2V REF
AVDD
REFLO
CURRENT
SOURCE
ARRAY
AVDD
REFIO
FS ADJ
R
SET AD9750
I
REF
V
GC
1mF
I
REF
= (1.2–V
GC
)/R
SET
WITH V
GC
< V
REFIO
AND 62.5mA # I
REF
# 625A
Figure 21. Dual-Supply Gain Control Circuit
ANALOG OUTPUTS
The AD9750 produces two complementary current outputs,
IOUTA and IOUTB, which may be configured for single-end
or differential operation. IOUTA and IOUTB can be converted
into complementary single-ended voltage outputs, V
OUTA
and
V
OUTB
, via a load resistor, R
LOAD
, as described in the DAC
Transfer Function section by Equations 5 through 8. The
differential voltage, V
DIFF
, existing between V
OUTA
and V
OUTB
can also be converted to a single-ended voltage via a transformer
or differential amplifier configuration.
Figure 22 shows the equivalent analog output circuit of the
AD9750 consisting of a parallel combination of PMOS differen-
tial current switches associated with each segmented current
source. The output impedance of IOUTA and IOUTB is deter-
mined 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., V
OUTA
and V
OUTB
)
and, to a lesser extent, the analog supply voltage, AVDD, and
full-scale current, I
OUTFS
. Although the output impedance’s
signal dependency can be a source of dc nonlinearity and ac linear-
ity (i.e., distortion), its effects can be limited if certain precau-
tions are noted.
AVDD
IOUTBIOUTA
R
LOAD
R
LOAD
Figure 22. Equivalent Analog Output
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 break-
down of the output stage and affect the reliability of the AD9750.
The positive output compliance range is slightly dependent on
the full-scale output current, I
OUTFS
. It degrades slightly from its
nominal 1.25 V for an I
OUTFS
= 20 mA to 1.00 V for an I
OUTFS
=
2 mA. Operation beyond the positive compliance range will
induce clipping of the output signal which severely degrades
the AD9750’s linearity and distortion performance.
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 AD9750
fixed, significantly reducing its effect on linearity. However,
it does not necessarily lead to the optimum distortion perfor-
mance due to limitations of the I-V op amp. Note that the
INL/DNL specifications for the AD9750 are measured in
this manner using IOUTA. In addition, these dc linearity
specifications remain virtually unaffected over the specified
power supply range of 4.5 V to 5.5 V.
Operating the AD9750 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 IOUTB 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 se-
lected transformer with a grounded center-tap will allow the
AD9750 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
R
LOAD
accordingly. Refer to Applying the AD9750 section for
examples of various output configurations.
The most significant improvement in the AD9750’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 wave-form’s
frequency content increases and/or its amplitude decreases.
The distortion and noise performance of the AD9750 is also
slightly dependent on the analog and digital supply as well as the
full-scale current setting, I
OUTFS
. 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. Although I
OUTFS
can be set between 2 mA and
20 mA, selecting an I
OUTFS
of 20 mA will provide the best dis-
tortion and noise performance also shown in Figure 8. The
noise performance of the AD9750 is affected by the digital sup-
ply (DVDD), output frequency, and increases with increasing
clock rate as shown in Figure 11. Operating the AD9750 with
low voltage logic levels between 3 V and 3.3 V will slightly re-
duce the amount of on-chip digital noise.
In summary, the AD9750 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.
(3) I
OUTFS
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 appro-
priate logic levels.
Note that the ac performance of the AD9750 is characterized
under the above mentioned operating conditions.
AD9750
–12– REV. 0
DIGITAL INPUTS
The AD9750’s digital input consists of 10 data input pins and a
clock input pin. The 10-bit parallel data inputs follow standard
positive binary coding where DB9 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 pulse-
width. 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,
V
THRESHOLD
set to approximately half the digital positive supply
(DVDD) or
V
THRESHOLD
= DVDD/2 (±20%)
The internal digital circuitry of the AD9750 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 V
OH(MAX)
. A DVDD of 3 V to 3.3 V will typically ensure
proper compatibility with most TTL logic families. Figure 23
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
AD9750 remains enabled if this input is left disconnected.
DVDD
DIGITAL
INPUT
Figure 23. Equivalent Digital Input
Since the AD9750 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. The drivers of the digital
data interface circuitry should be specified to meet the minimum
setup and hold times of the AD9750 as well as its required min/
max input logic level thresholds. Typically, the selection of
the slowest logic family that satisfies the above conditions will
result in the lowest data feedthrough and noise.
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
AD9750 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 in-
puts. Also, operating the AD9750 with reduced logic swings and
a corresponding digital supply (DVDD) will also reduce data
feedthrough.
The external clock driver circuitry should provide the AD9750
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 recon-
structed waveform. Thus, the clock input should be driven by
the fastest logic family suitable for the application.
Note, 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.
INPUT CLOCK/DATA TIMING RELATIONSHIP
SNR in a DAC is dependent on the relationship between the
position of the clock edges and the point in time at which the
input data changes. The AD9750 is positive edge triggered, and
so exhibits SNR sensitivity when the data transition is close to
this edge. In general, the goal when applying the AD9750 is to
make the data transitions shortly after the rising edge. This
becomes more important as the sample rate increases. Figure 24
shows the relationship of SNR to clock placement with different
sample rates and different frequencies out. Note that at the
lower sample rates, much more tolerance is allowed in clock
placement, while at higher rates, much more care must be taken.
TIME OF DATA CHANGE RELATIVE TO
RISING CLOCK EDGE – ns
40
–10 15–5 0 5 10
60
56
52
48
SNR – dB
44
F
S
= 65MSPS
F
S
= 125MSPS
Figure 24. SNR vs. Clock Placement 2 f
OUT
= 10 MHz
SLEEP MODE OPERATION
The AD9750 has a power-down function which 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 con-
tains an active pull-down circuit that ensures the AD9750 re-
mains enabled if this input is left disconnected. The AD9750
takes less than 50 ns to power down and approximately 5 µs to
power back up.
AD9750
–13–
REV. 0
POWER DISSIPATION
The power dissipation, P
D
, of the AD9750 is dependent on
several factors which include: (1) AVDD and DVDD, the
power supply voltages; (2) I
OUTFS
, the full-scale current output;
(3) f
CLOCK
, the update rate; (4) and the reconstructed digital
input waveform. The power dissipation is directly proportional
to the analog supply current, I
AVDD
, and the digital supply cur-
rent, I
DVDD
. I
AVDD
is directly proportional to I
OUTFS
as shown in
Figure 25 and is insensitive to f
CLOCK
.
Conversely, I
DVDD
is dependent on both the digital input wave-
form, f
CLOCK
, and digital supply DVDD. Figures 26 and 27
show I
DVDD
as a function of full-scale sine wave output ratios
(f
OUT
/f
CLOCK
) for various update rates with DVDD = 5 V and
DVDD = 3 V, respectively. Note, how I
DVDD
is reduced by more
than a factor of 2 when DVDD is reduced from 5 V to 3 V.
I
OUTFS
– mA
35
52204 6 8 1012141618
30
25
20
15
10
I
AVDD
– mA
Figure 25. I
AVDD
vs. I
OUTFS
RATIO (f
CLOCK
/f
OUT
)
18
16
00.01 10.1
I
DVDD
– mA
8
6
4
2
12
10
14
125MSPS
100MSPS
50MSPS
25MSPS
5MSPS
Figure 26. I
DVDD
vs. Ratio @ DVDD = 5 V
RATIO (f
CLOCK
/f
OUT
)
8
00.01 10.1
I
DVDD
– mA
6
4
2
125MSPS
100MSPS
50MSPS
25MSPS
5MSPS
Figure 27. I
DVDD
vs. Ratio @ DVDD = 3 V
APPLYING THE AD9750
OUTPUT CONFIGURATIONS
The following sections illustrate some typical output configura-
tions for the AD9750. Unless otherwise noted, it is assumed
that I
OUTFS
is set to a nominal 20 mA. For applications requir-
ing 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 opti-
mum 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 appropri-
ately sized load resistor, R
LOAD
, referred to ACOM. This con-
figuration may be more suitable for a single-supply system
requiring a dc coupled, ground referred output voltage. Alterna-
tively, 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.
DIFFERENTIAL COUPLING USING A TRANSFORMER
An RF transformer can be used to perform a differential-to-
single-ended signal conversion as shown in Figure 28. 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. Trans-
formers with different impedance ratios may also be used for
impedance matching purposes. Note that the transformer
provides ac coupling only.
AD9750
–14– REV. 0
R
LOAD
AD9750
MINI-CIRCUITS
T1-1T
OPTIONAL R
DIFF
IOUTA
IOUTB
Figure 28. Differential Output Using a Transformer
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 IOUTB. The complementary voltages
appearing at IOUTA and IOUTB (i.e., V
OUTA
and V
OUTB
) swing
symmetrically around ACOM and should be maintained with
the specified output compliance range of the AD9750. A differ-
ential resistor, R
DIFF
, may be inserted in applications in which
the output of the transformer is connected to the load, R
LOAD
,
via a passive reconstruction filter or cable. R
DIFF
is determined
by the transformer’s impedance ratio and provides the proper
source termination which results in a low VSWR. Note that
approximately half the signal power will be dissipated across R
DIFF
.
DIFFERENTIAL USING AN OP AMP
An op amp can also be used to perform a differential to single-
ended conversion as shown in Figure 29. The AD9750 is con-
figured with two equal load resistors, R
LOAD
, 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 amps distor-
tion performance by preventing the DACs high slewing output
from overloading the op amp’s input.
AD9750
IOUTA
IOUTB C
OPT
500V
225V
225V
500V
25V25V
AD8055
Figure 29. DC Differential Coupling Using an Op Amp
The common-mode rejection of this configuration is typically
determined by the resistor matching. In this circuit, the differ-
ential op amp circuit is configured to provide some additional
signal gain. The op amp must operate off of a dual supply since
its output is approximately ±1.0 V. A high speed amplifier such
as the AD8055 or AD8057 capable of preserving the differential
performance of the AD9750 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 opti-
mizing this circuit.
The differential circuit shown in Figure 30 provides the neces-
sary level-shifting required in a single supply system. In this
case, AVDD which is the positive analog supply for both the
AD9750 and the op amp is also used to level-shift the differ-
ential output of the AD9750 to midsupply (i.e., AVDD/2). The
AD8041 is a suitable op amp for this application.
AD9750
IOUTA
IOUTB C
OPT
500V
225V
225V
1kV
25V25V
AD8041
1kVAVDD
Figure 30. Single-Supply DC Differential Coupled Circuit
SINGLE-ENDED UNBUFFERED VOLTAGE OUTPUT
Figure 31 shows the AD9750 configured to provide a unipolar
output range of approximately 0 V to +0.5 V for a doubly termi-
nated 50 Ω cable since the nominal full-scale current, I
OUTFS
, of
20 mA flows through the equivalent R
LOAD
of 25 Ω. In this
case, R
LOAD
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 R
LOAD
.
Different values of I
OUTFS
and R
LOAD
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.
AD9750
IOUTA
IOUTB 50V
25V
50V
VOUTA = 0 TO +0.5V
IOUTFS = 20mA
Figure 31. 0 V to +0.5 V Unbuffered Voltage Output
SINGLE-ENDED, BUFFERED VOLTAGE OUTPUT
CONFIGURATION
Figure 32 shows a buffered single-ended output configuration in
which the op amp U1 performs an I-V conversion on the AD9750
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 distor-
tion performance at higher DAC update rates may be limited by
U1’s slewing capabilities. U1 provides a negative unipolar out-
put voltage and its full-scale output voltage is simply the
product of R
FB
and I
OUTFS
. The full-scale output should be set
within U1’s voltage output swing capabilities by scaling I
OUTFS
and/or R
FB
. An improvement in ac distortion performance may
result with a reduced I
OUTFS
since the signal current U1 will be
required to sink will be subsequently reduced.
AD9750
–15–
REV. 0
AD9750
IOUTA
IOUTB
C
OPT
200V
U1
V
OUT
= I
OUTFS
3 R
FB
I
OUTFS
= 10mA
R
FB
200V
Figure 32. Unipolar Buffered Voltage Output
POWER AND GROUNDING CONSIDERATIONS, POWER
SUPPLY REJECTION
Many applications seek high speed and high performance under
less than ideal operating conditions. In these circuits, the imple-
mentation and construction of the printed circuit board design
are as important as the circuit design. To ensure optimum per-
formance, proper RF techniques must be used for device selec-
tion, placement and routing as well as power supply bypassing
and grounding. Figures 39–44 illustrate the recommended
printed circuit board ground, power and signal plane layouts
which are implemented on the AD9750 evaluation board.
One factor that can measurably affect system performance is the
ability of the DAC output to reject dc variations or ac noise
superimposed on the analog or digital dc power distribution
(i.e., AVDD, DVDD). This is referred to as Power Supply
Rejection Ratio (PSRR). For dc variations of the power supply,
the resulting performance of the DAC directly corresponds to a
gain error associated with the DAC’s full-scale current, I
OUTFS
.
AC noise on the dc supplies is common in applications where
the power distribution is generated by a switching power supply.
Typically, switching power supply noise will occur over the
spectrum from tens of kHz to several MHz. PSRR vs. frequency
of the AD9750 AVDD supply, over this frequency range, is
given in Figure 33.
FREQUENCY – MHz
90
80
60 0.26 0.5
PSRR – dB
0.75 1.0
70
Figure 33. Power Supply Rejection Ratio of AD9750
Note that the units in Figure 33 are given in units of (amps out)/
(volts in). Noise on the analog power supply has the effect of
modulating the internal switches, and therefore the output
current. The voltage noise on the dc power will be added in a
nonlinear manner to the desired I
OUT
. Due to the relatively
different sizes of these switches, PSRR is very code-dependent.
This can produce a mixing effect which can modulate low fre-
quency power supply noise to higher frequencies. Worst case
PSRR for either one of the differential DAC outputs will occur
when the full-scale current is directed towards that output. As a
result, the PSRR measurement in Figure 33 represents a worst
case condition in which the digital inputs remain static and the
full-scale output current of 20 mA is directed to the DAC out-
put being measured.
An example serves to illustrate the effect of supply noise on the
analog supply. Suppose a switching regulator with a switching
frequency of 250 kHz produces 10 mV rms of noise and, for
simplicity sake (i.e., ignore harmonics), all of this noise is con-
centrated at 250 kHz. To calculate how much of this undesired
noise will appear as current noise superimposed on the DAC’s
full-scale current, I
OUTFS
, one must determine the PSRR in dB
using Figure 33 at 250 kHz. To calculate the PSRR for a given
R
LOAD
, such that the units of PSRR are converted from A/V to
V/V, adjust the curve in Figure 33 by the scaling factor 20 ×
Log (R
LOAD
). For instance, if R
LOAD
is 50 Ω, the PSRR is re-
duced by 34 dB (i.e., PSRR of the DAC at 1 MHz, which is
74 dB in Figure 33 becomes 40 dB V
OUT
/V
IN
).
Proper grounding and decoupling should be a primary objective
in any high speed, high resolution system. The AD9750 fea-
tures separate analog and digital supply and ground pins to
optimize the management of analog and digital ground currents
in a 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 that require 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 34. 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.
100mF
ELECT. 10-22mF
TANT. 0.1mF
CER.
TTL/CMOS
LOGIC
CIRCUITS
+5V OR +3V
POWER SUPPLY
FERRITE
BEADS
AVDD
ACOM
Figure 34. Differential LC Filter for Single +5 V or +3 V
Applications
Maintaining low noise on power supplies and ground is critical
to obtaining optimum results from the AD9750. If properly
implemented, ground planes can perform a host of functions on
high speed circuit boards: bypassing, shielding, current trans-
port, 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.
AD9750
–16– REV. 0
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 volt-
age drops in the signal ground paths. It is recommended that all
connections be short, direct and as physically close to the pack-
age 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 resistor
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.
APPLICATIONS
Using the AD9750 for Quadrature Amplitude Modulation
(QAM)
QAM is one of the most widely used digital modulation schemes in
digital communication systems. This modulation technique can
be found in FDM as well as spreadspectrum (i.e., CDMA)
based systems. A QAM signal is a carrier frequency that is
modulated in both amplitude (i.e., AM modulation) and phase
(i.e., PM modulation). It can be generated by independently
modulating two carriers of identical frequency but with a 90°
phase difference. This results in an in-phase (I) carrier compo-
nent and a quadrature (Q) carrier component at a 90° phase
shift with respect to the I component. The I and Q components
are then summed to provide a QAM signal at the specified
carrier frequency.
A common and traditional implementation of a QAM modu-
lator is shown in Figure 35. The modulation is performed in the
analog domain in which two DACs are used to generate the
baseband I and Q components, respectively. Each component is
then typically applied to a Nyquist filter before being applied to
a quadrature mixer. The matching Nyquist filters shape and
limit each component’s spectral envelope while minimizing
intersymbol interference. The DAC is typically updated at the
QAM symbol rate or possibly a multiple of it if an interpolating
filter precedes the DAC. The use of an interpolating filter typi-
cally eases the implementation and complexity of the analog
filter, which can be a significant contributor to mismatches in
gain and phase between the two baseband channels. A quadra-
ture mixer modulates the I and Q components with in-phase
and quadrature phase carrier frequency and then sums the two
outputs to provide the QAM signal.
AD9750
0
90
AD9750
CARRIER
FREQUENCY
12
12
TO
MIXER
DSP
OR
ASIC
NYQUIST
FILTERS QUADRATURE
MODULATOR
S
Figure 35. Typical Analog QAM Architecture
In this implementation, it is much more difficult to maintain
proper gain and phase matching between the I and Q channels.
The circuit implementation shown in Figure 36 helps improve
upon the matching and temperature stability characteristics
between the I and Q channels, as well as showing a path for up-
conversion using the AD8346 quadrature modulator. Using a
single voltage reference derived from U1 to set the gain for both
the I and Q channels will improve the gain matching and stabil-
ity. R
CAL
can be used to compensate for any mismatch in gain
between the two channels. This mismatch may be attributed to
the mismatch between R
SET1
and R
SET2
, effective load resistance
of each channel, and/or the voltage offset of the control ampli-
fier in each DAC. The differential voltage outputs of U1 and
U2 are fed into the respective differential inputs of the AD8346
via matching networks.
AD9750
–17–
REV. 0
It is also possible to generate a QAM signal completely in the
digital domain via a DSP or ASIC, in which case only a single
DAC of sufficient resolution and performance is required to
reconstruct the QAM signal. Also available from several vendors
are Digital ASICs which implement other digital modulation
schemes such as PSK and FSK. This digital implementation has
the benefit of generating perfectly matched I and Q components
in terms of gain and phase, which is essential in maintaining
optimum performance in a communication system. In this imple-
mentation, the reconstruction DAC must be operating at a
sufficiently high clock rate to accommodate the highest specified
QAM carrier frequency. Figure 37 shows a block diagram of
such an implementation using the AD9750.
50V
AD9750
LPF
50V
TO
MIXER
12 COS
12
SIN
12
12
I DATA
Q DATA
12
CARRIER
FREQUENCY
12
STEL-1177
NCO
CLOCK
STEL-1130
QAM
Figure 37. Digital QAM Architecture
AD9750 EVALUATION BOARD
General Description
The AD9750-EB is an evaluation board for the AD9750 10-bit
D/A converter. Careful attention to layout and circuit design
combined with a prototyping area allow the user to easily and
effectively evaluate the AD9750 in any application where high
resolution, high speed conversion is required.
This board allows the user the flexibility to operate the AD9750 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
on-board option to add a resistor network for proper load ter-
mination. Provisions are also made to operate the AD9750
with either the internal or external reference, or to exercise the
power-down feature.
Refer to the application note AN-420 for a thorough description
and operating instructions for the AD9750 evaluation board.
AD9750
(“I DAC”)
AD9750
(“Q DAC”)
IOUTA
IOUTB
QOUTA
QOUTB
DCOM
FSADJ
REFIO SLEEP
R
SET2
1.9kV
0.1mF
CLK
Q DATA
INPUT
I DATA
INPUT
DVDD
AVDD 100W
500V
100V
C
FILTER
100V
C
FILTER
100V
500V
500V
500V500V
500V500V
634V
0.1mF
+5V
VPBF
BBIP
BBIN
BBQP
BBQN
AD8346
PHASE
SPLITTER
LOIP
LOIN
VOUT
500mV p-p WITH
V
CM
=1.2V
NOTE: 500V RESISTOR NETWORK - OHMTEK ORN5000D
100V RESISTOR NETWORK - TOMC1603-100D
REFLO
ACOM
REFLO
AVDD
REFIO
FSADJ
R
SET1
2kV
R
CAL
220V
U1
U2
AVDD
1.82V
LATCHES 500V
DAC
DAC
+
LATCHES
Figure 36. Baseband QAM Implementation Using Two AD9750s
AD9750
–18– REV. 0
1098765432
1
R4
10
98765432
1
R7
DVDD
10
98765432
1
R3
1098765432
1
DVDD
R6
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
P1 10
98765432
1
R5
DVDD
10
98765432
1
R1
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
C19
C1
C2
C25
C26
C27
C28
C29
16 PINDIP
RES PK
16
15
14
13
12
11
10
1
2
3
4
5
6
7
C30
C31
C32
C33
C34
C35
C36
16 PINDIP
RES PK
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
DB13
DB12
DB11
DB10
DB9
DB8
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
CLOCK
DVDD
DCOM
NC
AVDD
COMP2
IOUTA
IOUTB
ACOM
COMP1
FS ADJ
REFIO
REFLO
SLEEP
U1
AD975x
AVDD
CT1
A1
A
R15
49.9V
CLK
JP1
AB
3
2
1
J1 TP1
EXTCLK
C7
1mFC8
0.1mF
AVDD
A
C9
0.1mF
TP8
2
AVDD
TP11
C11
0.1mF
TP10 TP9
R16
2kV
TP14
JP4
C10
0.1mF
OUT 1 OUT 2
TP13
R17
49.9V
PDIN
J2 A A A AVDD
3JP2
TP12
TP7
A
C6
10mF
AVCC
B6
TP6
A
C5
10mF
AVEE
B5
TP19
A
AGND
B4
TP18
TP5
C4
10mF
TP4
AVDD
B3
TP2
DGND
B2
C3
10mF
TP3
DVDD
B1
R20
49.9V
J3
C12
22pF
A A
R14
0
A
4
5
61
3
T1
J7
R38
49.9V
J4
A A
JP6A
JP6B
A
R13
OPEN
C13
22pF
C20
0
R12
OPEN
A
B
A
JP7B
B
A
JP7A
R10
1kV
B
A
JP8
R9
1kVA
B
A
R35
1kV
JP9
R18
1kV
A
376
24
AD8047
C21
0.1mF
A
C22
1mF
R36
1kV
C23
0.1mF
A
C24
1mF
AVEE
AVCC
R37
49.9V
J6
A
376
24
123
JP5
C15
0.1mF
A
AVEE
R46
1kV
C17
0.1mF
A
1
2
3
JP3
A
B
AVCC
A
CW
R43
5kV
R45
1kV
C14
1mF
A
R44
50V
EXTREFIN
J5
A
R42
1kV
C16
1mF
A
AVCC
C18
0.1mF
U7
6
2
4
A
VIN VOUT
GND
REF43
98765432
1
R2
10
A
1098765432
1
DVDD
R8
U6
A
AD8047
OUT2
OUT1 U4
Figure 38. Evaluation Board Schematic
AD9750
–19–
REV. 0
Figure 39. Silkscreen Layer—Top
Figure 40. Component Side PCB Layout (Layer 1)
AD9750
–20– REV. 0
Figure 41. Ground Plane PCB Layout (Layer 2)
Figure 42. Power Plane PCB Layout (Layer 3)
AD9750
–21–
REV. 0
Figure 43. Solder Side PCB Layout (Layer 4)
Figure 44. Silkscreen Layer—Bottom
AD9750
–22– REV. 0
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
28-Lead, 300 Mil SOIC
(R-28)
SEATING
PLANE
0.0118 (0.30)
0.0040 (0.10) 0.0192 (0.49)
0.0138 (0.35)
0.1043 (2.65)
0.0926 (2.35)
0.0500
(1.27)
BSC 0.0125 (0.32)
0.0091 (0.23)
0.0500 (1.27)
0.0157 (0.40)
88
08
0.0291 (0.74)
0.0098 (0.25) 3 458
0.7125 (18.10)
0.6969 (17.70)
0.4193 (10.65)
0.3937 (10.00)
0.2992 (7.60)
0.2914 (7.40)
PIN 1
28 15
14
1
28-Lead TSSOP
(RU-28)
28 15
14
1
0.386 (9.80)
0.378 (9.60)
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)
88
08
C3377–8–1/99
PRINTED IN U.S.A.
