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| 型號： | AD9768SE |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | DAC |
| 英文描述： | Ultrahigh Speed IC D/A Converter |
| 中文描述： | PARALLEL, 8 BITS INPUT LOADING, 0.005 us SETTLING TIME, 8-BIT DAC, CQCC20 |
| 封裝： | CERAMIC, LCC-20 |
| 文件頁數(shù)： | 3/4頁 |
| 文件大小： | 103K |
| 代理商： | AD9768SE |
AD9768
REV. A
–3–
speed, high performance device: optimum use requires careful
attention to all design details, including the layout of the circuit
in which the converter is used.
CONVE NT IONAL AD9768SD
Refer to Figure 1, Conventional AD9768SD.
T he output current of the AD9768 appears at Pin 13 (I
O
) and
develops a voltage across the load resistor R
L
which is based on:
A. I
M
(the current flowing through the single-transistor
source discussed above)
B. Value of R
L
Figure 1. Conventional AD9768SD
I
M
is a function of the return voltage (V
RET
), the reference
voltage (V
REF
), and the value of R
SET
; all of these are selected by
the user for his application. T he necessary equations for
calculating precise values for each are part of Figure 1. As
indicated, the voltage drop across R
L
is added to the return
voltage; the resulting voltage is the total V
OUT
of the converter.
VOLT AGE MULT IPLY ING MODE
In addition to its use as an ultra-high speed current output D/A
converter, the AD9768 can also be used as a two-quadrant
multiplying D/A in either a voltage mode or a current mode.
Refer to Figure 2, Multiplying AD9768 (Voltage Mode).
When operating in this mode, the analog output of the AD9768
is influenced by the digital inputs and an external multiplying
voltage (V
M
) applied to Pin 16 REFERENCE IN, which takes
the place of the internal reference used when the D/A is
operating in a conventional manner.
Figure 2. Multiplying AD9768 (Voltage Mode)
T he value of I
M
flowing through R
SET
is set by the voltage of
V
RET
minus the multiplying voltage (V
M
), divided by R
SET
; the
amount of this current is part of the equation which establishes
the analog output (V
OUT
) of the AD9768 and is chosen by the
user for his application. As it is when operating the D/A in a
conventional fashion, V
RET
can be any value between 0 volts and
+3 volts. V
M
(for purposes of discussion here) is some negative
voltage and can be varied over a range which is approximately 1
volt peak-to-peak.
T HE ORY OF OPE RAT ION
Refer to the AD9768SD schematic.
T he transistors pictured on the bottom of the diagram, con-
nected to paired transistors in the middle of the schematic, are
current sources which are always “on”. T he paired transistors
are differential current switches, designed to steer current from
the current sources to either Pin 13 (I
O
) or Pin 14 (
I
O
).
Digital inputs applied to Pins 1-8 determine which transistors
will be operating in each pair and establish what current will
flow at Pins 13 and 14.
T he transistor on the extreme left of the schematic is a base
reference for the paired current switches and is used to assure
the switches will be centered around an ECL voltage swing. T he
diodes connected to the base of this transistor are temperature
compensation devices for the base reference circuit.
T here are three different current sources in the AD9768 D/A.
T he eight transistors shown on the bottom of the schematic are
structured as two identical groups of four current sources, each
of which is binarily weighted. T he MSB group, comprised of the
four on the right, is connected to the LSB group through a 15:1
current divider made up of two 50
and two 750
resistor
networks. T he geometry of the AD9768 guarantees the binary
weighing ratios among the 100, 200, 400 and 800 resistors in
each emitter circuit are correct.
T he resistor values which are shown indicate the ratios among
the resistors, and not their nominal values.
T he third current source is a single transistor, pictured in the
lower left portion of the schematic with its collector connected
to Pin 18 R
SET
. Its function is to help establish the base voltage
on the eight current sources; it works in conjunction with the
external R
SET
resistor selected by the user of the AD9768, and
the reference amplifier. Current flowing through this transistor
is referred to as I
M
in the figures and text.
When the AD9768 is operating as a conventional current-output
D/A converter, I
M
develops a voltage across R
SET
which is one of
the inputs to the on-board reference amplifier shown in the
schematic. T he other input to this amplifier is the on-chip
reference voltage of –1.26 volts.
T he output of the reference amplifier adjusts the current-source
base reference voltage at Pin 17; this, in turn, adjusts the value
of I
M
in the single-transistor current source and causes it to
develop a voltage across R
SET
which maintains Pin 18 at the
–1.26 volts of the on-chip reference supply.
T o maintain good stability in the internal loop reference
amplifier, a ceramic chip capacitor with a nominal value of
0.01
μ
F should be connected to Pin 17 COMPENSAT ION;
minimum recommended value for this capacitor is 3900 pF.
T he temperature coefficient of the load resistor (R
L
) can affect
the performance of the AD9768 D/A converter, as it can with
any current-output converter. T he design and use of the
AD9768 and its dependence on an external R
SET
resistor, how-
ever, make it sensitive also to the tempco of R
SET
. T he user is
cautioned to select R
L
and R
SET
resistors which have low tem-
perature coefficients.
DIGIT AL GROUND (Pin 11) and ANALOG RET URN (Pin
12) are normally connected together; this connection should be
made as close as possible to the device case to minimize possible
noise problems. T he AD9768 D/A is similar to any other high-
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