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DAC908U Schematic ( PDF Datasheet ) - Burr-Brown Corporation

Teilenummer DAC908U
Beschreibung 8-Bit/ 165MSPS DIGITAL-TO-ANALOG CONVERTER
Hersteller Burr-Brown Corporation
Logo Burr-Brown Corporation Logo 




Gesamt 15 Seiten
DAC908U Datasheet, Funktion
®
DAC908
DAC908
For most current data sheet and other product
information, visit www.burr-brown.com
DAC908
TM 8-Bit, 165MSPS
DIGITAL-TO-ANALOG CONVERTER
FEATURES
q SINGLE +5V OR +3V OPERATION
q HIGH SFDR: 5.04MHz Output at 100MSPS: 67dBc
q LOW GLITCH: 3pV-s
q LOW POWER: 170mW at +5V
q INTERNAL REFERENCE:
Optional Ext. Reference
Adjustable Full-Scale Range
Multiplying Option
APPLICATIONS
q MEDICAL INSTRUMENTATION
Ultrasound (DBF)
q VIDEO, DIGITAL TV
q WAVEFORM GENERATION
Direct Digital Synthesis (DDS)
Arbitrary Waveform Generation (ARB)
q TEST INSTRUMENTATION
q COMMUNICATIONS
DESCRIPTION
The DAC908 is a high-speed, digital-to-analog converter (DAC)
offering an 8-bit resolution option within the SpeedPlus family
of high-performance converters. Featuring pin compatibility
among family members, the DAC900, DAC902, and DAC904
provide a component selection option to an 10-, 12-, and 14-bit
resolution, respectively. All models within this family of D/A
converters support update rates in excess of 165MSPS with
excellent dynamic performance, and are especially suited to
fulfill the demands of a variety of applications.
The advanced segmentation architecture of the DAC908 is
optimized to provide a high Spurious-Free Dynamic Range
(SFDR) for single-tone, as well as for multi-tone signals—
essential when used for the transmit signal path of communica-
tion systems.
The DAC908 has a high impedance (200k) current output with
a nominal range of 20mA and an output compliance of up to
1.25V. The differential outputs allow for both a differential, or
single-ended analog signal interface. The close matching of the
current outputs ensures superior dynamic performance in the
differential configuration, which can be implemented with a
transformer.
Utilizing a small geometry CMOS process, the monolithic
DAC908 can be operated on a wide, single-supply range of
+2.7V to +5.5V. Its low power consumption allows for use in
portable and battery operated systems. Further optimization can
be realized by lowering the output current with the adjustable
full-scale option.
For noncontinuous operation of the DAC908, a power-down
mode results in only 45mW of standby power.
The DAC908 comes with an integrated 1.24V bandgap refer-
ence and edge-triggered input latches, offering a complete
converter solution. Both +3V and +5V CMOS logic families
can be interfaced to the DAC908.
The reference structure of the DAC908 allows for additional
flexibility by utilizing the on-chip reference, or applying an
external reference. The full-scale output current can be adjusted
over a span of 2mA to 20mA, with one external resistor, while
maintaining the specified dynamic performance.
The DAC908 is available in the SO-28 and TSSOP-28 pack-
ages.
+VA
DAC908
FSA
REFIN
INT/EXT
BW +VD
Current
Sources
LSB
Switches
Segmented
Switches
IOUT
IOUT
BYP
+1.24V Ref.
AGND
CLK
Latches
PD
8-Bit Data Input
D7...D0
DGND
International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111
Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
©1999 Burr-Brown Corporation
PDS-11507B
DAC908Printed in U.S.A. April, 2000
®






DAC908U Datasheet, Funktion
TYPICAL PERFORMANCE CURVES VD = VA = +5V
At TA = +25°C, differential transformer coupled output, 50doubly terminated, and SFDR up to Nyquist, unless otherwise noted.
85
80
75
70
65
60
55
0
SFDR vs fOUT AT 25MSPS
–6dBFS
0dBFS
2.0 4.0 6.0 8.0 10.0 12.0
Frequency (MHz)
85
80
75
70
65
60
55
0
SFDR vs fOUT AT 50MSPS
–6dBFS
0dBFS
5.0 10.0 15.0 20.0 25.0
Frequency (MHz)
85
80
75
70
65
60
55
50
45
0
SFDR vs fOUT AT 100MSPS
–6dBFS
0dBFS
10.0 20.0 30.0 40.0
Frequency (MHz)
50.0
85
80
75
70
65
60
55
50
45
0
SFDR vs fOUT AT 125MSPS
–6dBFS
0dBFS
10.0 20.0 30.0 40.0 50.0 60.0
Frequency (MHz)
80
75
70
65
60
55
50
45
40
0
SFDR vs fOUT AT 165MSPS
–6dBFS
0dBFS
10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0
Frequency (MHz)
80
75
70
65
60
55
50
45 X
40
2
SFDR vs IOUTFS and fOUT AT 100MSPS
2.1MHz
5.04MHz
10.1MHz
20.2MHz
X 40.4MHz
X
X
5 10
IOUTFS (mA)
20
®
DAC908
6

6 Page









DAC908U pdf, datenblatt
As shown in Figure 3, the transformer’s center tap must be
connected to ground to enable the necessary dc-current flow
for both outputs. Some applications may require a solid
termination, in which case a differential resistor, RDIFF, may
be inserted as shown. Note that this will reduce the available
signal power by approximately one half.
IOUT
DAC908
IOUT
Optional
RDIFF
ADT1-1WT
(Mini-Circuits)
1:1
50
50
RL
FIGURE 3. Differential Output Configuration Using an RF
Transformer.
DIFFERENTIAL CONFIGURATION USING AN OP AMP
If the application requires a dc-coupled output, a difference
amplifier may be considered, as shown in Figure 4. Four
external resistors are needed to configure the voltage-feed-
back op amp OPA680 as a difference amplifier performing
the differential to single-ended conversion. Under the shown
configuration, the DAC908 generates a differential output
signal of 0.5Vp-p at the load resistors, RL. The resistor
values shown were selected to result in a symmetric 25
loading for each of the current outputs since the input
impedance of the difference amplifier is in parallel to resis-
tors RL, and should be considered.
IOUT
DAC908
IOUT
R1
200
CDIFF
RL
26.1
R3
200
RL
28.7
R2
402
OPA680
–5V +5V
R4
402
VOUT
FIGURE 4. Difference Amplifier Provides Differential to
Single-Ended Conversion and DC-Coupling.
The OPA680 is configured for a gain of two. Therefore,
operating the DAC908 with a 20mA full-scale output will
produce a voltage output of ±1V. This requires the amplifier
to operate off of a dual power supply (±5V). The tolerance
of the resistors typically sets the limit for the achievable
common-mode rejection. An improvement can be obtained
by fine tuning resistor R4.
This configuration typically delivers a lower level of ac
performance than the previously discussed transformer solu-
tion because the amplifier introduces another source of
distortion. Suitable amplifiers should be selected based on
their slew-rate, harmonic distortion, and output swing capa-
bilities. High-speed amplifiers like the OPA680 or OPA687
may be considered. The ac performance of this circuit may
be improved by adding a small capacitor, CDIFF, between the
outputs IOUT and IOUT, as shown in Figure 4. This will intro-
duce a real pole to create a low-pass filter in order to slew-
limit the DACs fast output signal steps, which otherwise
could drive the amplifier into slew-limitations or into an
overload condition; both would cause excessive distortion.
The difference amplifier can easily be modified to add a
level shift for applications requiring the single-ended output
voltage to be unipolar, i.e., swing between 0V and +2V.
DUAL TRANSIMPEDANCE OUTPUT CONFIGURATION
The circuit example of Figure 5 shows the signal output
currents connected into the summing junction of the
OPA2680, which is set up as a transimpedance stage, or
‘I to V converter’. With this circuit, the DAC’s output will
be kept at a virtual ground, minimizing the effects of output
impedance variations, and resulting in the best dc linearity
(INL). However, as mentioned previously, the amplifier
may be driven into slew-rate limitations, and produce un-
wanted distortion. This may occur, especially, at high DAC
update rates.
+5V
50
1/2
OPA2680
DAC908
IOUT
CD1
IOUT
CD2
RF1
CF1
RF2
CF2
1/2
OPA2680
50
–5V
–VOUT = IOUT • RF
–VOUT = IOUT • RF
FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680
Forms Differential Transimpedance Amplifier.
®
DAC908
12

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