a
FEATURES
Member of Pin-Compatible TxDAC Product Family
125 MSPS Update Rate
8-Bit Resolution
Linearity: 1/4 LSB DNL
Linearity: 1/4 LSB INL
Differential Current Outputs
SINAD @ 5 MHz Output: 50 dB
Power Dissipation: 175 mW @ 5 V to 45 mW @ 3 V
Power-Down Mode: 20 mW @ 5 V
On-Chip 1.20 V Reference
Single +5 V or +3 V Supply Operation
Packages: 28-Lead SOIC and 28-Lead TSSOP
Edge-Triggered Latches
Fast Settling: 35 ns Full-Scale Settling to 0.1%
APPLICATIONS
Communications
Signal Reconstruction
Instrumentation
PRODUCT DESCRIPTION
The AD9708 is the 8-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, was specifically optimized for the transmit signal path of communication systems. All
of the devices share the same interface options, small outline
package and pinout, thus providing an upward or downward
component selection path based on performance, resolution and
cost. The AD9708 offers exceptional ac and dc performance
while supporting update rates up to 125 MSPS.
The AD9708’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 45 mW, without a significant degradation in
performance, by lowering the full-scale current output. In addition, a power-down mode reduces the standby power dissipation to approximately 20 mW.
The AD9708 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 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 AD9708 is a current-output DAC with a nominal full-scale
output current of 20 mA and > 100 kΩ output impedance.
TxDAC is a registered trademark of Analog Devices, Inc.
REV. B
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.
8-Bit, 100 MSPS+
TxDAC® D/A Converter
AD9708
FUNCTIONAL BLOCK DIAGRAM
+5V
0.1mF
REFLO
COMP1
AVDD
+1.20V REF
0.1mF
50pF
REF IO
CURRENT
SOURCE
ARRAY
FS ADJ
ACOM
AD9708
COMP2
0.1mF
RSET
+5V
CLOCK
IOUTA
DVDD
DCOM
SEGMENTED
SWITCHES
CLOCK
IOUTB
LATCHES
SLEEP
DIGITAL DATA INPUTS (DB7–DB0)
Differential current outputs are provided to support singleended or differential applications. The current outputs may be
directly tied to an output resistor to provide two complementary, single-ended voltage outputs. The output voltage compliance
range is 1.25 V.
The AD9708 contains a 1.2 V on-chip reference and reference
control amplifier, which allows the full-scale output current to
be simply set by a single resistor. The AD9708 can be driven by
a variety of external reference voltages. The AD9708’s full-scale
current can be adjusted over a 2 mA to 20 mA range without
any degradation in dynamic performance. Thus, the AD9708
may operate at reduced power levels or be adjusted over a 20 dB
range to provide additional gain ranging capabilities.
The AD9708 is available in 28-lead SOIC and 28-lead TSSOP
packages. It is specified for operation over the industrial temperature range.
PRODUCT HIGHLIGHTS
1. The AD9708 is a member of the TxDAC product family, which
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 AD9708 uses a proprietary switching technique that enhances dynamic performance well beyond 8- and 10-bit video DACs.
3. On-chip, edge-triggered input CMOS latches readily interface
to +3 V and +5 V CMOS logic families. The AD9708 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 AD9708 to operate at reduced power levels
(i.e., 45 mW) without any degradation in dynamic performance.
5. A temperature compensated, 1.20 V bandgap reference is
included on-chip providing a complete DAC solution. An
external reference may be used.
6. The current output(s) of the AD9708 can easily be configured for various single-ended or differential applications.
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
AD9708–SPECIFICATIONS
DC SPECIFICATIONS (T
MIN
to TMAX , AVDD = +5 V, DVDD = +5 V, IOUTFS = 20 mA, unless otherwise noted)
Parameter
Min
RESOLUTION
8
MONOTONICITY
Typ
Max
Units
Bits
GUARANTEED OVER SPECIFIED TEMPERATURE RANGE
1
DC ACCURACY
Integral Linearity Error (INL)
Differential Nonlinearity (DNL)
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
± 1/4
± 1/4
–1/2
–1/2
–0.025
–10
–10
2.0
–1.0
OPERATING RANGE
LSB
LSB
+0.025
+10
+10
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
±2
±1
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 Ratio—AVDD
Power Supply Rejection Ratio—DVDD
+1/2
+1/2
2.7
2.7
5.0
5.0
25
3
–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
140
190
45
5.5
5.5
30
6
8.5
175
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.
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 = 50 MSPS and fOUT = 1.0 MHz.
7
Measured as unbuffered voltage output into 50 Ω RLOAD at IOUTA and IOUTB, f CLOCK = 100 MSPS and fOUT = 40 MHz.
Specifications subject to change without notice.
–2–
REV. B
AD9708
(TMIN to TMAX , AVDD = +5 V, DVDD = +5 V, IOUTFS = 20 mA, Single-Ended Output, IOUTA, 50 ⍀ Doubly
DYNAMIC SPECIFICATIONS Terminated, unless otherwise noted)
Parameter
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 (I OUTFS = 20 mA)
Output Noise (I OUTFS = 2 mA)
AC LINEARITY TO NYQUIST
Signal-to-Noise and Distortion Ratio
fCLOCK = 10 MSPS; fOUT = 1.00 MHz
fCLOCK = 50 MSPS; fOUT = 1.00 MHz
fCLOCK = 50 MSPS; fOUT = 12.51 MHz
fCLOCK = 100 MSPS; fOUT = 5.01 MHz
fCLOCK = 100 MSPS; fOUT = 25.01 MHz
Total Harmonic Distortion
fCLOCK = 10 MSPS; fOUT = 1.00 MHz
fCLOCK = 50 MSPS; fOUT = 1.00 MHz
fCLOCK = 50 MSPS; fOUT = 12.51 MHz
fCLOCK = 100 MSPS; fOUT = 5.01 MHz
fCLOCK = 100 MSPS; fOUT = 25.01 MHz
Spurious-Free Dynamic Range to Nyquist
fCLOCK = 10 MSPS; fOUT = 1.00 MHz
fCLOCK = 50 MSPS; fOUT = 1.00 MHz
fCLOCK = 50 MSPS; fOUT = 12.51 MHz
fCLOCK = 100 MSPS; fOUT = 5.01 MHz
fCLOCK = 100 MSPS; fOUT = 25.01 MHz
Min
Typ
100
125
35
1
5
2.5
2.5
50
30
MSPS
ns
ns
pV-s
ns
ns
pA/√Hz
pA/√Hz
50
50
48
50
45
dB
dB
dB
dB
dB
–67
–67
–59
–64
–48
Max
–62
68
68
63
67
50
62
Units
dBc
dBc
dBc
dBc
dBc
dBc
dBc
dBc
dBc
dBc
NOTES
1
Measured single ended into 50 Ω load.
Specifications subject to change without notice.
DIGITAL SPECIFICATIONS (T
MIN
to TMAX, AVDD = +5 V, DVDD = +5 V, IOUTFS = 20 mA unless otherwise noted)
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 (t LPW)
Min
Typ
3.5
2.1
5
3
0
0
2.0
1.5
3.5
ns
5
Specifications subject to change without notice.
DB0–DB7
tS
tH
tLPW
tPD
tST
IOUTA
OR
IOUTB
0.1%
0.1%
Figure 1. Timing Diagram
REV. B
–3–
Units
V
V
V
V
µA
µA
pF
ns
ns
–10
–10
CLOCK
Max
1.3
0.9
+10
+10
AD9708
ABSOLUTE MAXIMUM RATINGS*
PIN FUNCTION DESCRIPTIONS
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
Pin No. Name
Description
1
2–7
8
9–14, 25
15
DB7
DB6–DB1
DB0
NC
SLEEP
16
REFLO
17
REFIO
18
19
FS ADJ
COMP1
28-Lead 300 mil SOIC
θJA = 71.4°C/W
θJC = 23°C/W
20
21
ACOM
IOUTB
28-Lead TSSOP
θJA = 97.9°C/W
θJC = 14.0°C/W
22
IOUTA
23
COMP2
24
AVDD
26
27
DCOM
DVDD
28
CLOCK
Most Significant Data Bit (MSB).
Data Bits 1–6.
Least Significant Data Bit (LSB).
No Internal Connection.
Power-Down Control Input. Active
High. Contains active pull-down circuit,
thus 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).
Digital Common.
Digital Supply Voltage (+2.7 V to
+5.5 V).
Clock Input. Data latched on positive
edge of clock.
Parameter
AVDD
DVDD
ACOM
AVDD
CLOCK, SLEEP
Digital Inputs
IOUTA, IOUTB
COMP1, COMP2
REFIO, FSADJ
REFLO
Junction Temperature
Storage Temperature
Lead Temperature
(10 sec)
–65
*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 effect device reliability.
THERMAL CHARACTERISTICS
Thermal Resistance
PIN CONFIGURATION
28 CLOCK
(MSB) DB7 1
DB6 2
27 DVDD
DB5 3
26 DCOM
DB4 4
25 NC
DB3 5
DB2 6
AD9708
24 AVDD
TOP VIEW 23 COMP2
DB1 7 (Not to Scale) 22 IOUTA
DB0 8
21 IOUTB
NC 9
20 ACOM
NC 10
19 COMP1
NC 11
18 FS ADJ
NC 12
17 REFIO
NC 13
16 REFLO
NC 14
15 SLEEP
ORDERING GUIDE
Model
Temperature
Range
Package
Descriptions
Package
Options*
AD9708AR –40°C to +85°C 28-Lead 300 Mil SOIC R-28
AD9708ARU –40°C to +85°C 28-Lead TSSOP
RU-28
AD9708-EB Evaluation Board
NC = NO CONNECT
*R = Small Outline IC; RU = Thin Small Outline IC.
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 AD9708 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
AD9708
DEFINITIONS OF SPECIFICATIONS
Linearity Error (Also Called Integral Nonlinearity or INL)
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.
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.
Power Supply Rejection
The maximum change in the full-scale output as the supplies
are varied from nominal to minimum and maximum specified
voltages.
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.
Settling Time
Monotonicity
A D/A converter is monotonic if the output either increases or
remains constant as the digital input increases.
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.
Offset Error
Glitch Impulse
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.
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.
Gain Error
The difference, in dB, between the rms amplitude of the output
signal and the peak spurious signal over the specified bandwidth.
Spurious-Free Dynamic Range
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.
Signal-to-Noise and Distortion (S/N+D, SINAD) Ratio
S/N+D is the ratio of the rms value of the measured output
signal to the rms sum of all other spectral components below the
Nyquist frequency, including harmonics but excluding dc. The
value for S/N+D is expressed in decibels.
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.
Total Harmonic Distortion
THD is the ratio of the rms sum 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).
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
+5V
0.1mF
REFLO
+1.20V REF
0.1mF
REF IO
COMP1
AVDD
AD9708
CURRENT
SOURCE
ARRAY
FS ADJ
RSET
2kV
+5V
ACOM
50pF
COMP2
0.1mF
IOUTA
DVDD
SEGMENTED
SWITCHES
DCOM
IOUTB
CLOCK
50V
LATCHES
DVDD
DCOM
50V
RETIMED
CLOCK
OUTPUT*
LECROY 9210
PULSE GENERATOR
SLEEP
50V
CLOCK
OUTPUT
DIGITAL
DATA
TEKTRONIX
AWG-2021
Figure 2. Basic AC Characterization Test Setup
REV. B
–5–
20pF
TO HP3589A
SPECTRUM/
NETWORK
ANALYZER
50V INPUT
20pF
* AWG2021 CLOCK RETIMED
SUCH THAT DIGITAL DATA
TRANSITIONS ON FALLING EDGE
OF 50% DUTY CYCLE CLOCK.
AD9708
Typical AC Characterization Curves (AVDD = +5 V or +3 V, DVDD = +5 V or +3 V, 50 ⍀ Doubly Terminated Load,
Single-Ended Output, IOUTA, IOUTFS = 20 mA, TA = +25ⴗC, unless otherwise noted)
70
70
55
THD @ 10MSPS
THD @ 50MSPS
THD @ 10MSPS
55
SINAD @ 10MSPS
SINAD @ 50MSPS
45
THD @ 50MSPS
55
50
10
1
FREQUENCY – MHz
30
100
Figure 4. SINAD/THD vs. f OUT (Differential Output, AVDD and DVDD = 5.0 V)
THD
@ 100MSPS
60
THD @ 50MSPS
55
SINAD @ 10MSPS
IOUTFS = 20mA
THD
@ 50MSPS
50 IOUTFS = 10mA
IOUTFS = 5mA
55
THD @ 100MSPS
50
100
10
FREQUENCY – MHz
60
48
IOUTFS = 2.5mA
46
SINAD @ 10MSPS
44
45
SINAD @ 100MSPS
SINAD @ 100MSPS
1
10
FREQUENCY – MHz
1
Figure 5. SINAD vs. IOUTFS
@ 100 MSPS
SINAD @ 50MSPS
SINAD @ 50MSPS
40
0.1
IOUTFS = 2.5mA
52
THD
@ 10MSPS
65
SINAD/THD – dB
SINAD/THD – dB
10
1
FREQUENCY – MHz
70
45
40
SINAD @ 100MSPS
40
0.1
100
70
50
IOUTFS = 5mA
35
SINAD @ 50MSPS
45
Figure 3. SINAD/THD vs. fOUT (AVDD
and DVDD = 5.0 V)
65 THD @ 10MSPS
45
SINAD @ 10MSPS
SINAD @ 100MSPS
40
0.1
IOUTFS = 10mA
60
SINAD – dB
50
50
THD
@ 100MSPS
SINAD – dB
60
IOUTFS = 20mA
65
THD
@ 100MSPS
SINAD/THD – dB
SINAD/THD – dB
65
40
0.1
100
Figure 6. SINAD/THD vs. fOUT (AVDD
and DVDD = 3.0 V)
1
10
FREQUENCY – MHz
42
0.1
100
Figure 7. SINAD/THD vs. f OUT (Differential Output, AVDD and DVDD = 3.0 V)
Figure 8. SINAD vs. IOUTFS
@ 20 MSPS
0
0
0.6
fCLOCK = 25MSPS
fOUT = 7.81MHz
SFDR = +60.7dBc
AMPLITUDE = 0dBFS
fCLOCK = 125MSPS
fOUT = 27.0MHz
SFDR = +52.7dBc
AMPLITUDE = 0dBc
0.5
0.4
0.3
VOLTS
10dB – Div
10dB – Div
10
1
FREQUENCY – MHz
0.2
0.1
0.0
–0.1
–100
–100
START: 0Hz
STOP: 12.5MHz
Figure 9. Single-Tone Spectral Plot
@ 25 MSPS
–0.2
START: 0Hz
STOP: 62.5MHz
Figure 10. Single-Tone Spectral
Plot @ 125 MSPS
–6–
TIME – 5ns/Div
Figure 11. Step Response
REV. B
AD9708
FUNCTIONAL DESCRIPTION
As previously mentioned, 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:
Figure 12 shows a simplified block diagram of the AD9708. The
AD9708 consists of a large PMOS current source array 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 remaining 3 LSBs are also implemented with equally
weighted current sources whose sum total equals 7/8th of an
MSB current source. Implementing the upper and lower bits
with current sources 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.
IOUTFS = 32 × IREF
(3)
where
IREF = VREFIO /RSET
(4)
The two current outputs will typically drive a resistive load
directly. If dc coupling is required, IOUTA and IOUTB should
be directly connected to matching resistive loads, RLOAD, which
are tied to analog common, ACOM. Note, 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:
The analog and digital sections of the AD9708 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.
VOUTA = IOUTA × R LOAD
(5)
V OUTB = IOUTB × R LOAD
(6)
Note the full-scale value of VOUTA and VOUTB should not exceed
the specified output compliance range to maintain specified
distortion and linearity performance.
The differential voltage, VDIFF , appearing across IOUTA and
IOUTB is:
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 thirty-two times the value of IREF.
Substituting the values of IOUTA , IOUTB, and IREF; VDIFF can be
expressed as:
DAC TRANSFER FUNCTION
VOLTAGE REFERENCE AND CONTROL AMPLIFIER
The AD9708 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 = 255), while
IOUTB, the complementary output, provides no current. The
current output appearing at IOUTA and IOUTB are a function
of both the input code and IOUTFS and can be expressed as:
The AD9708 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 an external reference is selected. If
REFLO is tied to ACOM, as shown in Figure 13, 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. Note that REFIO is not designed to drive any external load. It should be buffered with an external amplifier
having an input bias current less than 100 nA if any additional
loading is required.
IOUTA = (DAC CODE/256) × IOUTFS
VDIFF = (IOUTA – IOUTB) × RLOAD
VDIFF = {(2 DAC CODE – 255)/256}/ × (32 RLOAD /RSET)
× VREFIO
(1)
IOUTB = (255 – DAC CODE)/256 × IOUTFS
(2)
where DAC CODE = 0 to 255 (i.e., Decimal Representation).
+5V
0.1mF
REFLO
+1.20V REF
VREFIO
REFIO
IREF
0.1mF
RSET
2kV
ACOM
AD9708
50pF
CURRENT
SOURCE
ARRAY
FS ADJ
COMP2
DVDD
IOUTA
SEGMENTED
SWITCHES
IOUTB
CLOCK
LATCHES
SLEEP
DIGITAL DATA INPUTS (DB7–DB0)
Figure 12. Functional Block Diagram
REV. B
0.1mF
VDIFF = VOUTA – VOUTB
+5V
DCOM
CLOCK
AVDD
COMP1
–7–
IOUTA
IOUTB
VOUTA
VOUTB
RLOAD
50V
RLOAD
50V
(7)
(8)
AD9708
+5V
The small signal bandwidth of the reference control amplifier is
approximately 1.8 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. If IREF is fixed for an application, a 0.1 µF ceramic chip
capacitor is recommended.
0.1mF
OPTIONAL
EXTERNAL
REF BUFFER
REFLO
COMP1
+1.2V REF
50pF
REFIO
ADDITIONAL
LOAD
0.1mF
CURRENT
SOURCE
ARRAY
FS ADJ
2kV
AVDD
AD9708
Figure 13. Internal Reference Configuration
IREF can be varied for a fixed RSET by disabling the internal
reference and varying the common-mode voltage 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 R-2R ladder DAC configured in
the voltage mode topology may be used to control the gain. This
circuit is shown in Figure 15 using the AD7524 and an external
1.2 V reference, the AD1580. Note another AD9708 could also
be used as the gain control DAC since it can also provide a
programmable unipolar output up to 1.2 V.
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 14. 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.
AVDD
ANALOG OUTPUTS AND OUTPUT CONFIGURATIONS
The AD9708 produces two complementary current outputs,
IOUTA and IOUTB , which may be converted into complementary
single-ended voltage outputs, VOUTA and VOUTB, via a load resistor,
RLOAD, as described in the DAC TRANSFER FUNCTION
section. Figure 16 shows the AD9708 configured to provide a
positive unipolar output range of approximately 0 V to +0.5 V
for a double terminated 50 Ω cable for a nominal full-scale
current, IOUTFS, of 20 mA. In this case, RLOAD represents the
equivalent load resistance seen by IOUTA or IOUTB and is
equal to 25 Ω. The unused output (IOUTA or IOUTB) can be
connected to ACOM directly or via a matching RLOAD. Different
values of IOUTFS and RLOAD can be selected as long as the positive compliance range is adhered to.
0.1mF
REFLO
AVDD
COMP1
AVDD
+1.2V REF
50pF
VREFIO
EXTERNAL
REF
REFIO
CURRENT
SOURCE
ARRAY
FS ADJ
RSET
IREF =
VREFIO/RSET
REFERENCE
CONTROL
AMPLIFIER
AD9708
Figure 14. External Reference Configuration
The AD9708 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 14, such that its current output, IREF, is determined by
the ratio of the VREFIO and an external resistor, RSET, as stated
in Equation 4. 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 provides several application benefits. The first benefit
relates directly to the power dissipation of the AD9708, 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.
AD9708
IOUTFS = 20mA
VOUTA = 0 TO +0.5V
IOUTA 22
50V
50V
IOUTB 21
25V
Figure 16. 0 V to +0.5 V Unbuffered Voltage Output
Alternatively, an amplifier could be configured as an I-V converter
thus converting IOUTA or IOUTB into a negative unipolar
AVDD
OPTIONAL
BANDLIMITING
CAPACITOR
AVDD
REFLO
RFB
1.2V
AD1580
OUT1
+1.2V REF
VDD
AD7524
VREF
0.1V TO 1.2V
OUT2
COMP1
50pF
REFIO
FS ADJ
AGND
RSET
IREF =
VREF/RSET
AVDD
CURRENT
SOURCE
ARRAY
AD9708
DB7–DB0
Figure 15. Single-Supply Gain Control Circuit
–8–
REV. B
AD9708
voltage. Figure 17 shows a buffered singled-ended output configuration in which the op amp, U1, performs an I-V conversion
on the AD9708 output current. U1 provides a negative unipolar
output voltage and its full-scale output voltage is simply the
product of RFB and IOUTFS. 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 and will be subsequently reduced. Note, the ac
distortion performance of this circuit at higher DAC update
rates may be limited by U1’s slewing capabilities.
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, VOH(MAX) ,
of the TTL drivers. A DVDD of 3 V to 3.3 V will typically
ensure upper compatibility of most TTL logic families.
DVDD
DIGITAL
INPUT
COPT
RFB
200V
AD9708
Figure 18. Equivalent Digital Input
IOUTFS = 10mA
IOUTA 22
U1
VOUT = IOUTFS 3 RFB
IOUTB 21
200V
Figure 17. Unipolar Buffered Voltage Output
IOUTA and IOUTB also have a negative and positive voltage
compliance range that must be adhered to in order to achieve
optimum performance. The positive output compliance range is
slightly dependent on the full-scale output current, IOUTFS. It
degrades slightly from its nominal 1.25 V for an IOUTFS = 20 mA
to 1.00 V for an IOUTFS = 2 mA. Applications requiring the
AD9708’s output (i.e., VOUTA and/or VOUTB) to extend up to its
output compliance range should size RLOAD accordingly. Operation
beyond this compliance range will adversely affect the AD9708’s
linearity.
The differential voltage, VDIFF , existing between VOUTA and
VOUTB may also be converted to a single-ended voltage via a
transformer or differential amplifier configuration. Refer to the
DIFFERENTIAL OUTPUT CONFIGURATION section for
more information.
DIGITAL INPUTS
The AD9708’s digital input consists of eight data input pins and
a clock input pin. The 8-bit parallel data inputs follow standard
positive binary coding where DB7 is the most significant bit
(MSB) and DB0 is the least significant bit (LSB). 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.
The digital inputs are CMOS compatible with logic thresholds,
VTHRESHOLD set to approximately half the digital positive supply
(DVDD) or
VTHRESHOLD = DVDD/2 (±20%)
Figure 18 shows the equivalent digital input circuit for the data
and clock inputs. The sleep mode input is similar, except that
it contains an active pull-down circuit, thus ensuring that the
AD9708 remains enabled if this input is left disconnected. The
internal digital circuitry of the AD9708 is capable of operating
REV. B
Since the AD9708 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 AD9708 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 AD9708
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. Also,
operating the AD9708 with reduced logic swings and a corresponding digital supply (DVDD) will also reduce data feedthrough.
The external clock driver circuitry should provide the AD9708
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. However, the clock input could also be
driven by via a sine wave, which is centered around the digital
threshold (i.e., DVDD/2), and meets the min/max logic threshold.
This may result in a slight degradation in the phase noise, which
becomes more noticeable at higher sampling rates and output
frequencies. Note, 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.
SLEEP MODE OPERATION
The AD9708 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 AD9708 remains enabled if
this input is left disconnected. The SLEEP input with active
pull-down requires
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