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PDF AD668 Data sheet ( Hoja de datos )
| Número de pieza | AD668 | |
| Descripción | 12-Bit Ultrahigh Speed Multiplying D/A Converter | |
| Fabricantes | Analog Devices | |
| Logotipo |  |
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FEATURES
Ultrahigh Speed: Current Settling to 1 LSB in 90 ns for
a Full-Scale Change in Digital Input. Voltage Settling
to 1 LSB in 120 ns for a Full-Scale Change in Analog
Input
15 MHz Reference Bandwidth
Monotonicity Guaranteed over Temperature
10.24 mA Current Output or 1.024 V Voltage Output
Integral and Differential Linearity Guaranteed over
Temperature
0.3" “Skinny DIP” Packaging
MIL-STD-883 Compliant Versions Available
12-Bit Ultrahigh Speed
Multiplying D/A Converter
AD668
FUNCTIONAL BLOCK DIAGRAM
PRODUCT DESCRIPTION
The AD668 is an ultrahigh speed, 12-bit, multiplying digital-to-
analog converter, providing outstanding accuracy and speed per-
formance in responding to both analog and digital inputs. The
AD668 provides a level of performance and functionality in a
monolithic device that exceeds that of many contemporary hy-
brid devices. The part is fabricated using Analog Devices’
Complementary Bipolar (CB) Process, which features vertical
NPN and PNP devices on the same chip without the use of
dielectric isolation. The AD668’s design capitalizes on this pro-
prietary process in combination with standard low impedance
circuit techniques to provide its unique combination of speed
and accuracy in a monolithic part.
The wideband reference input is buffered by a high gain, closed
loop reference amplifier. The reference input is essentially a 1 V,
high impedance input, but trimmed resistive dividers are pro-
vided to readily accommodate 5 V and 1.25 V references. The
reference amplifier features an effective small signal bandwidth
of 15 MHz and an effective slew rate of 3% of full scale/ns.
Multiple matched current sources and thin film ladder tech-
niques are combined to produce bit weighting. The output range
can nominally be taken as a 10.24 mA current output or a 1.024 V
voltage output. Varying the analog input can provide modulation
of the DAC full scale from 10% to 120% of its nominal value.
Bipolar outputs can be realized through pin-strapping to provide
two-quadrant operation without additional external circuitry.
Laser wafer trimming insures full 12-bit linearity and excellent
gain accuracy. All grades of the AD668 are guaranteed mono-
tonic over their full operating temperature range. Furthermore,
the output resistance of the DAC is trimmed to 100 Ω ± 1.0%.
The AD668 is available in four performance grades. The
AD668JQ and KQ are specified for operation from 0°C to
+70°C, the AD668AQ is specified for operation from –40°C to
+85°C, and the AD668SQ specified for operation from –55°C
to +125°C. All grades are available in a 24-pin cerdip (0.3"
package.
PRODUCT HIGHLIGHTS
1. The fast settling time of the AD668 provides suitable perfor-
mance for waveform generation, graphics display, and high
speed A/D conversion applications.
2. The high bandwidth reference channel allows high frequency
modulation between analog and digital inputs.
3. The AD668’s design is configured to allow wide variation of
the analog input, from 10% to 120% of its nominal value.
4. The AD668’s combination of high performance and tremen-
dous flexibility makes it an ideal building block for a variety
of high speed, high accuracy instrumentation applications.
5. The digital inputs are readily compatible with both TTL and
5 V CMOS logic families.
6. Skinny DIP (0.3") packaging minimizes board space require-
ments and eases layout considerations.
7. The AD668 is available in versions compliant with MIL-
STD-883. Refer to the Analog Devices Military Products
Databook or current AD668/883B data sheet for detailed
specifications.
REV. A
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
1 page  
ANALOG OFFSET ERROR: The analog offset is defined as
the offset of the analog amplifier channel, referred to the analog
input. Ideally, this would be measured with the analog input at
0 V and the digital input at full scale. Since a 0 V analog input
voltage constitutes an undervoltage condition, this specification
is determined through linear extrapolation, as indicated in
Figure 1.
Figure 1. Derivation of Analog Offset Voltage
GLITCH IMPULSE: Asymmetrical switching times in a DAC
may give rise to undesired output transients which are quanti-
fied by their glitch impulse. It is specified as the net area of the
glitch in pV-sec.
AD668
In current output mode:
Unipolar Mode
IOUT
= V IN
V NOM
×
DAC code
4096
× 10.24 mA
Bipolar Mode
IOUT
= V IN
V NOM
×
DAC code
4096
× 10.24 mA – V IN
V NOM
× 5.12 mA
In voltage output mode:
VOUT = IOUT × RLOAD
(for both unipolar and bipolar modes)
where:
VIN – the analog input voltage.
VNOM – the nominal full scale of the reference voltage: 1 V,
1.25 V, or 5 V, determined by the wiring configuration of Pins
21 and 22. (See APPLYING THE AD668.)
DAC code – the numerical representation of the DAC’s digital
inputs; a number between 0 and 4095.
RLOAD – the resistance of the DAC output node; the maximum
this can be is 200 Ω (the internal DAC ladder resistance). The
on-board load resistor (Pin 19) has been trimmed so that its
parallel combination with the DAC ladder resistance is 100 Ω
(± 1%)
Bipolar mode – produces a bipolar analog output from the digital
input by offsetting the normal output current with a precision
current source. This offset is achieved by connecting Pin 16 to
the DAC output. In the unipolar mode, Pin 16 should be
grounded.
If the dc errors are included, the transfer function becomes
somewhat more complex:
IOUT
=
VIN
VNOM
DAC code
+ OFFSET ANALOG × 4096 × (1 + E ) × 10.24 mA
Figure 2. AD668 Major Carry Glitch
FUNCTIONAL DESCRIPTION
The AD668 is designed to combine excellent performance with
maximum flexibility. The functional block diagram and the
simple transfer functions provided below will provide the user
with a basic grasp of the AD668’s operation. Examples of typi-
cal circuit configurations are provided in the section APPLY-
ING THE AD668. Subsequent sections contain more detailed
information useful in optimizing DAC performance in high
speed, high resolution applications.
DAC Transfer Function
The AD668 may be used either in a current output mode (DAC
output connected to a virtual ground) or a voltage output mode
(DAC output connected to a resistive load).
+ OFFSET DIGITAL × VIN × 10.24 mA
VNOM
–
V IN
V NOM
+ OFFSET ANALOG
× (5.12 mA + [OFFSET BIPOLAR
× 10.24 mA])
(Last term is for use in bipolar mode; VOUT is still just IOUT ×
RLOAD)
where:
OFFSETANALOG = the analog offset error.
OFFSETDIGITAL = is the unipolar digital offset error.
OFFSETBIPOLAR = is the bipolar offset error.
E = the gain error, expressed fractionally.
Operating Limits:
REV. A
–5–
5 Page 
reference input. While the two DACs are similar in many ways,
the optimal decoupling schemes differ between the two parts
and care should be used to insure that each is implemented
appropriately.
AD668
Foil Side
Figure 14. PC Board Layout
Figure 13. AD568 Driving the AD668
CONSTRUCTION GUIDELINES
HIGH FREQUENCY PRINTED CIRCUIT BOARD
SUGGESTIONS
In systems seeking to simultaneously achieve high speed and
high accuracy, the implementation and construction of the cir-
cuit is often as important as the circuit’s design. Proper RF
techniques must be used in device selection, placement and
routing, and supply bypassing and grounding. In many areas,
the performance of the AD668 may exceed the measurement ca-
pabilities of common lab instruments, making performance
evaluation particularly difficult. The AD668 has been config-
ured to be relatively easy to use in spite of these problems, and
realization of the performance indicated in this datasheet should
not be difficult if proper care is taken. Figure 14 provides an il-
lustration of the printed circuit board layout used for much of
the AD668’s characterization. The board represents an imple-
mentation of the circuit shown in Figure 23, with the AD586
used to drive the reference channel (as in Figure 11).
THE USE OF GROUND AND POWER PLANES
If properly implemented, ground planes can perform a myriad
of functions on high speed circuit boards: bypassing, shielding,
current transport, etc. In mixed signal design, the analog and
digital portions of the board should be distinct from one an-
other, with the analog ground plane confined to areas covering
analog signal traces and the digital ground plane confined to
areas covering digital interconnect. The two ground planes
should be connected by paths 1/4 inch to 1/2 inch wide on both
sides of the DAC, as shown in Figure 14. Care should be taken
to insure that the ground plane is uninterrupted over crucial sig-
nal 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 analog input signal, the DAC output sig-
nal, and the supply feeders. The use of wide runs or planes in
the routing of the power supplies is also recommended. This
serves the dual function of providing a low series impedance
power supply to the part as well as providing some “free” ca-
pacitive decoupling to the appropriate ground plane.
USING THE RIGHT BYPASS CAPACITORS
The capacitors used to bypass the power supplies are probably
the most important external components in any high speed de-
sign. Both selection and placement of these capacitors can be
critical and, to a large extent, dependent upon the specifics of
the system configuration. The dominant consideration in the
selection of bypass capacitors for the AD668 is minimization of
series resistance and inductance. Many capacitors will begin to
look inductive at 20 MHz and above. Ceramic and film type
capacitors generally feature lower series inductance than tanta-
lum or electrolytic types. A few general rules are of universal use
when approaching the problem of bypassing.
Bypass capacitors should be installed on the printed circuit
board with the shortest possible leads consistent with reliable
construction. This helps to minimize series inductance in the
leads. Chip capacitors are optimal in this respect.
Some series inductance between the DAC supply pins and the
power supply plane may help to filter-out high frequency power
supply noise. This inductance can be generated using a small
ferrite bead.
REV. A
Component Side
–11–
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet AD668.PDF ] | |
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