DAC0830LCM National Semiconductor, DAC0830LCM Datasheet - Page 14

• Manufacturer Part Number: DAC0830LCM
• Description: 8-Bit P Compatible/ Double-Buffered D to A Converters
• Manufacturer: National Semiconductor

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DAC0830 Series Application Hints
2.7 Using the DAC0830 in a Voltage Switching
Configuration
The R-2R ladder can also be operated as a voltage switch-
ing network. In this mode the ladder is used in an inverted
manner from the standard current switching configuration.
This configuration offers several useful application advan-
tages. Since the output is a voltage, an external op amp is
not necessarily required but the output impedance of the
DAC is fairly high (equal to the specified reference input re-
sistance of 10 k
buffering purposes. Some of the advantages of this mode
are illustrated in Figures 12, 13, 14, 15 .
There are two important things to keep in mind when using
this DAC in the voltage switching mode. The applied refer-
ence voltage must be positive since there are internal para-
sitic diodes from ground to the I
which would turn on if the applied reference went negative.
There is also a dependence of conversion linearity and gain
error on the voltage difference between V
applied to the normal current output terminals. This is a re-
sult of the voltage drive requirements of the ladder switches.
To ensure that all 8 switches turn on sufficiently (so as not to
add significant resistance to any leg of the ladder and
thereby introduce additional linearity and gain errors) it is
recommended that the applied reference voltage be kept
less than +5V
V
gain error change. Figures 16, 17, 18 characterize the ef-
fects of bringing V
typical temperature performance of this voltage switching
configuration.
(Continued)
REF
. These restrictions ensure less than 0.1% linearity and
DC
and V
to 20 k ) so an op amp may be used for
REF
CC
and V
be at least 9V more positive than
CC
closer together as well as
OUT1
and I
FIGURE 10. Adding Full-Scale Adjustment
CC
FIGURE 11. Voltage Mode Switching
and the voltage
OUT2
terminals
14
The reference voltage is connected to one of the current out-
put terminals (I
complementary binary) and the output voltage is taken from
the normal V
in the range from 0V to 255/256 V
plied digital code as shown in Figure 11 .
• Voltage switching mode eliminates output signal inver-
• Zero code output voltage is limited by the low level output
•
sion and therefore a need for a negative power supply.
saturation voltage of the op amp. The 2 k
sistor helps to reduce this voltage.
V
OS
of the op amp has no effect on DAC linearity.
REF
FIGURE 12. Single Supply DAC
OUT1
DS005608-11
pin. The converter output is now a voltage
for true binary digital control, I
REF
DS005608-12
as a function of the ap-
pull-down re-
OUT2
DS005608-41
is for