DAC08
Figure 26. Positive Low Impedance
Output Operation
Figure 27. Negative Low Impedance
Output Operation
Figure 28. Interfacing With Various Logic Families
APPLICATIONS INFORMATION
REFERENCE AMPLIFIER SET-UP
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:
I
FR
=
255
×
I
REF
, where I
REF
= I
14
.
256
Bipolar references may be accommodated by offsetting V
REF
or
pin 15. The negative common-mode range of the reference am-
plifier is given by: V
CM
– = V– plus (I
REF
×
1 kΩ) 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 rec-
ommended. 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 I
REF
and
I
FS
will eliminate the need for trimming I
REF
. 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 common-
mode range. The recommended range for operation with a dc
reference current is +0.2 mA to +4.0 mA.
In positive reference applications, an external positive reference
voltage forces current through R14 into the V
REF(+)
terminal
(pin 14) of the reference amplifier. Alternatively, a negative ref-
erence may be applied to V
REF(–)
at pin 15; reference current
flows from ground through R14 into V
REF(+)
as in the positive
reference case. This negative reference connection has the ad-
vantage of a very high impedance presented at pin 15. The volt-
age 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
eliminated with only a minor increase in error.
REV. A
–9–
AD [ ANALOG DEVICES ]
