Specifications

DAC1006/1007/1008—Simple Hookup for a “Quick Look” (Continued)
Operation is summarized in the table below:
Applied
2’s Comp. 2’s Comp. Applied True Binary V
OUT
(Decimal) (Binary) Digital Input (Decimal) +V
REF
−V
REF
[phone] 1111111111 1023 V
REF
−1 LSB −|V
REF
|+1 LSB
[phone] 1100000000 768 V
REF
/2 −|V
REF
|/2
0 0000000000 1000000000 512 0 0
−1 1111111111 0111111111 511 −1 LSB +1 LSB
−256 1100000000 0100000000 256 −V
REF
/2 +|V
REF
|/2
−512 1000000000 0000000000 0 −V
REF
+|V
REF
|
5.2 Analog Operation in the Voltage Switching Mode
Some useful application circuits result if the R-2R ladder is
operated in the voltage switching mode. There are two very
important things to remember when using the DAC in the
voltage mode. The reference voltage (+V) must always be
positive since there are parasitic diodes to ground on the
I
OUT1
pin which would turn on if the reference voltage went
negative. To maintain a degradation of linearity less than
±
0.005
%
, keep +V ≤ 3V
DC
and V
CC
at least 10V more posi-
tive than +V.
Figures 6, 7 show these errors for the voltage
switching mode. This operation appears unusual, since a ref-
erence voltage (+V) is applied to the I
OUT1
pin and the volt-
age output is the V
REF
pin. This basic idea is shown in Figure
8
.
This V
OUT
range can be scaled by use of a non-inverting
gain stage as shown in
Figure 9 .
Notice that this is unipolar operation since all voltages are
positive. A bipolar output voltage can be obtained by using a
single op amp as shown in
Figure 10 . For a digital input code
of all zeros, the output voltage from the V
REF
pin is zero
volts. The external op amp now has a single input of +V and
is operating with a gain of −1 to this input. The output of the
op amp therefore will be at −V for a digital input of all zeros.
As the digital code increases, the output voltage at the V
REF
pin increases.
Notice that the gain of the op amp to voltages which are ap-
plied to the (+) input is +2 and the gain to voltages which are
applied to the input resistor, R, is −1. The output voltage of
the op amp depends on both of these inputs and is given by:
V
OUT
=
(+V) (−1)+V
REF
(+2)
DS005688-41
FIGURE 4. Providing a Unipolar Output Voltage
DS005688-42
FIGURE 5. Providing a Bipolar Output Voltage with the DAC in the Current Switching Mode
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PrintDate=1998/11/17 PrintTime=11:38:09 46711 ds005688 Rev. No. 4 cmserv Proof 11