DAC08 Analog Devices, DAC08 Datasheet - Page 13

• Manufacturer Part Number: DAC08
• Description: 8-Bit, High Speed, Multiplying D/A Converter (Universal Digital Logic Interface)
• Manufacturer: Analog Devices

Specifications of DAC08

• Resolution (bits): 8bit
• Dac Update Rate: 11.8MSPS
• Dac Settling Time: 85ns
• Max Pos Supply (v): +18V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

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
reference 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
where I
In positive reference applications, an external positive reference
voltage forces current through R14 into the V
(Pin 14) of the reference amplifier. Alternatively, a negative
reference may be applied to V
flows from ground through R14 into V
reference case. This negative reference connection has the
advantage 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 (nomi-
nally equal to R14) is used to cancel bias current errors; R15
may be eliminated with only a minor increase in error.
Bipolar references may be accommodated by offsetting V
Pin 15. The negative common-mode range of the reference
amplifier is given by 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 bypas-
sed to ground with a 0.1 µF capacitor.
For most applications, the tight relationship between I
eliminates the need for trimming I
trimming can be accomplished by adjusting the value of R14, or
by using a potentiometer for R14. An improved method of full-
scale trimming that eliminates potentiometer T.C. effects is
shown in the recommended full-scale adjustment circuit
(Figure 27).
Using lower values of reference current reduces negative power
supply current and increases reference amplifier negative
common-mode range. The recommended range for operation
with a dc reference current is 0.2 mA to 4.0 mA.
I
FR
REF
=
= I
255
256
14
×
I
REF
CM
– = V− plus (I
REF(–)
REF.
at Pin 15; reference current
If required, full-scale
REF(+)
REF
× 1 kΩ) plus 2.5 V.
as in the positive
REF(+)
terminal
REF
REF
and I
or
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FS
REFERENCE AMPLIFIER COMPENSATION FOR
MULTIPLYING APPLICATIONS
AC reference applications 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 kΩ, 2.5 kΩ, and 5.0 kΩ, minimum values
of C
proportionately increased values of C
so the ratio of C
For fastest response to a pulse, low values of R14 enabling small
C
such as a transistor current source, none of the preceding values
suffice, and the amplifier must be heavily compensated, which
decreases overall bandwidth and slew rate. For R14 = 1 kΩ and
C
transition from I
Operation with pulse inputs to the reference amplifier can 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 (I
occurs in 120 ns when the equivalent impedance at Pin 14 is
200 Ω and C
which is relatively independent of the R
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
possible 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
The logic threshold may be adjusted over a wide range by placing
an appropriate voltage at the logic threshold control pin (Pin 1,
V
over the temperature range, with V
For TTL and DTL interface, simply ground Pin 1. When
interfacing ECL, an I
other logic families, see Figure 32. For general set-up of the
logic control circuit, note that Pin 1 sources 100 µA typical;
external circuitry should be designed to accommodate this
current.
C
C
LC
values should be used. If Pin 14 is driven by a high impedance
= 15 pF, the reference amplifier slews at 4 mA/µs, enabling a
). Figure 16 shows the relationship between V
C
V− + (I
are 15 pF, 37 pF, and 75 pF. Larger values of R14 require
REF
= 0) condition. Full-scale transition (0 mA to 2 mA)
REF
C
= 0. This yields a reference slew rate of 16 mA/µs,
× 1 kΩ) + 2.5 V
C
REF
(pF) to R14 (kΩ) = 15.
= 0 to I
REF
= 1 mA is recommended. For interfacing
REF
= 2 mA in 500 ns.
TH
C
nominally 1.4 above V
for proper phase margin,
IN
and V
IN
LC
values.
and V
DAC08
TH
LC
.