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參數(shù)資料
| 型號： | DAC729 |
| 英文描述： | DAC729 - DISCONTINUED PRODUCT. No longer recommended for new design. |
| 中文描述： | DAC729 -已停產(chǎn)產(chǎn)品。不再推薦用于新設(shè)計。 |
| 文件頁數(shù)： | 5/12頁 |
| 文件大小： | 251K |
| 代理商： | DAC729 |
DAC729
5
THEORY OF OPERATION
The DAC729 is an 18-bit digital-to-analog converter sys-
tem, including a precision reference, low noise, fast settling
operational amplifier, and an 18-bit current source/DAC
chip contained in a hermetic 40-pin ceramic dual-in-line
package. Refer to Figure 11 for a schematic diagram of the
DAC729.
THE INTERNAL REFERENCE
The reference consists of a very low temperature coefficient
closed-loop reference zener circuit that has been tempera-
ture-drift-compensated by laser-trimming a zener current to
achieve less than 1ppm/
°
C temperature drift of V
REF
.
By strapping pin 32 (Reference Out) to pin 31 (Reference
In), the DAC will be properly biased from the internal
reference. The internal reference may be fine adjusted using
pin 35 as shown in Figure 7. The reference has an output
buffer that will supply 4mA for use external to the DAC729.
This load must remain constant because changing load on
the reference may change the reference current to the DAC.
In systems where several components need to track the same
system reference, the DAC729 may be used with an external
10V reference, however, the internal reference has lower
noise (6
μ
Vp-p) and better stability than other references
available.
THE OPERATIONAL AMPLIFIER
To support a DAC of this accuracy, the operational amplifier
must have a maximum gain-induced error of less than
1/3LSB, independent of output swing (the op amp must be
linear!) To support 15 bits (1/2-bit linearity), the op amp
must have a gain of 130,000V/V. For 18 bits, the minimum
DAC ANALOG OUTPUT
20V FSR
DIGITAL INPUT
COB
CSB
10V FSR
00 0000 0000 0000 0000 + Full Scale 9.999924V + Full Scale 9.999962V
11 1111 1111 1111 1111 – Full Scale
–10V
– Full Scale
0V
TABLE I. Digital Input Coding.
BURN-IN SCREENING
Burn-in screening is an option available for the DAC729
family of products. Burn-in duration is 160 hours at 100
°
C
(or equivalent combination of time and temperature).
All units are tested after burn-in to ensure that grade speci-
fications are met.
gain is well over 500,000V/V. Since thermal feedback is the
major limitation of gain for mono op amps, the amplifier
was designed as a high gain, fast settling mono op amp,
followed by a monolithic, unity-gain current buffer to isolate
the thermal effects of external loads from the input stage
gain transistors. The op amp and buffer are separated from
the DAC chip, minimizing thermally-induced linearity er-
rors in the DAC circuit. The op amp, like the reference, is
not dedicated to the DAC729. The user may want to add a
network, or select a different amplifier. The DAC729 inter-
nal op amp is intended to be the best choice for accuracy,
settling time, and noise.
THE DAC CHIP
The heart of the DAC729 is a monolithic current source and
switch integrated circuit. The absolute linearity, differential
linearity, and the temperature performance of the DAC729
are the result of the design, which utilizes the excellent
element matching of the current sources and switch transis-
tors to each other, and the tracking of the current setting
resistors to the feed back resistors. Older discrete designs
cannot achieve the performance of this monolithic DAC
design.
The two most significant bits are binarily weighted inter-
digitated current sources. The currents for bits 3 through 18
are scaled with both current source weighting and an R-2R
ladder. The circuit design is optimized for low noise and low
superposition error, with the current sources arranged to
minimize both code-dependent thermal errors and IR drop
errors. As a result, the superposition errors are typically less
than 20
μ
V.
The DAC chip is biased from a servo amplifier feeding into
the base line of the current sources. This servo amplifier sets
the collector current to be mirrored and scaled in the DAC
chip current sources, as shown in Figure 11. The reference
current for the servo is established by the reference voltage
applied to pin 31 feeding an internal resistor (20k
) to the
virtual ground of the servo amplifier.
DISCUSSION
OF SPECIFICATIONS
DIGITAL INPUT CODES
The DAC729 accepts complementary digital input codes in
either binary format (CSB for Unipolar or COB for Bipolar;
see Table I).
ELECTROSTATIC
DISCHARGE SENSITIVITY
Any integral circuit can be damaged by ESD. Burr-Brown
recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
ESD damage can range from subtle performance degrada-
tion to complete device failure. Precision integrated circuits
may be more susceptible to damage because very small
parametric changes could cause the device not to meet
published specifications.
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