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DAC08Q2/883C View Datasheet(PDF) - Analog Devices

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Description
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DAC08Q2/883C Datasheet PDF : 12 Pages
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DAC08
APPLICATION INFORMATION
REFERENCE AMPLIFIER SETUP
The DAC08 is a multiplying D/A converter in which the output
current is the product of a digital number and the input refer-
ence current. The reference current may be fixed or may vary
from nearly zero to 4.0 mA. The full-scale output current is a
linear function of the reference current and is given by:
IFR
=
255
256
×
IREF ,
where
IREF
=
I14
In positive reference applications, an external positive reference
voltage forces current through R14 into the VREF(+) terminal
(Pin 14) of the reference amplifier. Alternatively, a negative
reference may be applied to VREF() at Pin 15; reference current
flows from ground through R14 into VREF(+) as in the positive
reference case. This negative reference connection has the advan-
tage of a very high impedance presented at Pin 15. The voltage
at Pin 14 is equal to and tracks the voltage at Pin 15 due to the
high gain of the internal reference amplifier. R15 (nominally equal
to R14) is used to cancel bias current errors; R15 may be elimi-
nated with only a minor increase in error.
Bipolar references may be accommodated by offsetting VREF or
Pin 15. The negative common-mode range of the reference
amplifier is given by: VCM= Vplus (IREF × 1 k) plus 2.5 V.
The positive common-mode range is V+ less 1.5 V.
When a dc reference is used, a reference bypass capacitor is
recommended. A 5.0 V TTL logic supply is not recommended
as a reference. If a regulated power supply is used as a reference,
R14 should be split into two resistors with the junction bypassed to
ground with a 0.1 µF capacitor.
For most applications the tight relationship between IREF and IFS
will eliminate the need for trimming IREF. If required, full-scale
trimming may be accomplished by adjusting the value of R14, or
by using a potentiometer for R14. An improved method of
full-scale trimming which eliminates potentiometer T.C. effects
is shown in the recommended full-scale adjustment circuit.
Using lower values of reference current reduces negative power
supply current and increases reference amplifier negative com-
mon-mode range. The recommended range for operation with
a dc reference current is 0.2 mA to 4.0 mA.
REFERENCE AMPLIFIER COMPENSATION FOR
MULTIPLYING APPLICATIONS
AC reference applications will require the reference amplifier to
be compensated using a capacitor from Pin 16 to V. The value
of this capacitor depends on the impedance presented to Pin 14:
for R14 values of 1.0, 2.5 and 5.0 k, minimum values of CC
are 15, 37 and 75 pF. Larger values of R14 require proportion-
ately increased values of CC for proper phase margin, so the
ratio of CC (pF) to R14 (k) = 15.
For fastest response to a pulse, low values of R14 enabling
small CC values should be used. If Pin 14 is driven by a high
impedance such as a transistor current source, none of the
above values will suffice and the amplifier must be heavily
compensated which will decrease overall bandwidth and slew
rate. For R14 = 1 kand CC = 15 pF, the reference amplifier
slews at 4 mA/µs enabling a transition from IREF = 0 to IREF =
2 mA in 500 ns.
Operation with pulse inputs to the reference amplifier may be
accommodated by an alternate compensation scheme. This
technique provides lowest full-scale transition times. An internal
clamp allows quick recovery of the reference amplifier from a
cutoff (IREF = 0) condition. Full-scale transition (0 mA to 2 mA)
occurs in 120 ns when the equivalent impedance at Pin 14 is
200 and CC = 0. This yields a reference slew rate of 16 mA/µs,
which is relatively independent of RIN and VIN values.
LOGIC INPUTS
The DAC08 design incorporates a unique logic input circuit
that enables direct interface to all popular logic families and
provides maximum noise immunity. This feature is made pos-
sible by the large input swing capability, 2 µA logic input
current and completely adjustable logic threshold voltage.
For V= 15 V, the logic inputs may swing between 10 V
and +18 V. This enables direct interface with 15 V CMOS
logic, even when the DAC08 is powered from a 5 V supply.
Minimum input logic swing and minimum logic threshold
voltage are given by: Vplus (IREF × 1 k) plus 2.5 V. The
logic threshold may be adjusted over a wide range by placing
an appropriate voltage at the logic threshold control pin (Pin 1,
VLC). The appropriate graph shows the relationship between
VLC and VTH over the temperature range, with VTH nominally
1.4 above VLC. For TTL and DTL interface, simply ground pin
1. When interfacing ECL, an IREF = 1 mA is recommended. For
interfacing other logic families, see preceding page. For general
set-up of the logic control circuit, it should be noted that Pin 1
will source 100 µA typical; external circuitry should be designed
to accommodate this current.
Fastest settling times are obtained when Pin 1 sees a low imped-
ance. If Pin 1 is connected to a 1 kdivider, for example, it
should be bypassed to ground by a 0.01 µF capacitor.
ANALOG OUTPUT CURRENTS
Both true and complemented output sink currents are provided
where IO + IO = IFS. Current appears at the true(IO) output
when a 1(logic high) is applied to each logic input. As the
binary count increases, the sink current at pin 4 increases pro-
portionally, in the fashion of a positive logicD/A converter.
When a 0is applied to any input bit, that current is turned
off at Pin 4 and turned on at Pin 2. A decreasing logic count
increases IO as in a negative or inverted logic D/A converter.
Both outputs may be used simultaneously. If one of the outputs
is not required, it must be connected to ground or to a point
capable of sourcing IFS; do not leave an unused output pin open.
Both outputs have an extremely wide voltage compliance enabling
fast direct current-to-voltage conversion through a resistor tied
to ground or other voltage source. Positive compliance is 36 V
above Vand is independent of the positive supply. Negative
compliance is given by Vplus (IREF × 1 k) plus 2.5 V.
The dual outputs enable double the usual peak-to-peak load
swing when driving loads in quasi-differential fashion. This
feature is especially useful in cable driving, CRT deflection and
in other balanced applications such as driving center-tapped
coils and transformers.
POWER SUPPLIES
The DAC08 operates over a wide range of power supply voltages
from a total supply of 9 V to 36 V. When operating at supplies
of ± 5 V or less, IREF 1 mA is recommended. Low reference
current operation decreases power consumption and increases
negative compliance, reference amplifier negative common-mode
–10–
REV. B
 

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