dac1008 National Semiconductor Corporation, dac1008 Datasheet - Page 10

• Manufacturer Part Number: dac1008
• Description: Up Compatible, Double-buffered D To A Converters
• Manufacturer: National Semiconductor Corporation

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 (
positive since there are parasitic diodes to ground on the
I
negative To maintain a degradation of linearity less than
positive than
voltage switching mode This operation appears unusual
since a reference voltage (
and the voltage output is the V
shown in Figure 8
This V
gain stage as shown in Figure 9
g
OUT1
0 005% keep
pin which would turn on if the reference voltage went
OUT
range can be scaled by use of a non-inverting
a
V Figures 6 and 7 show these errors for the
a
V
s
FIGURE 9 Amplifying the Voltage Mode Output (Single Supply Operation)
3 V
FIGURE 6
a
DC
V) is applied to the I
and V
REF
pin This basic idea is
CC
a
V) must always be
at least 10V more
FIGURE 8 Voltage Mode Switching
OUT1
DIGITAL INPUT CODE
pin
10
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
zero volts The external op amp now has a single input of
output of the op amp therefore will be at
input of all zeros As the digital code increases the output
voltage at the V
Notice that the gain of the op amp to voltages which are
applied to the (
which are applied to the input resistor R is
voltage of the op amp depends on both of these inputs and
is given by
a
V and is operating with a gain of
V
OUT
REF
a
) input is
e
pin increases
(
a
FIGURE 7
V) (
b
a
1)
2 and the gain to voltages
a
V
REF
b
1 to this input The
(
a
b
2)
b
V for a digital
1 The output
REF
TL H 5688 – 12
pin is