dac1008 National Semiconductor Corporation, dac1008 Datasheet - Page 9

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

shown in Figure 4 where the sign of the reference voltage
has been changed to provide a positive output voltage Note
that the output current I
pin
5 1 2 Providing a Bipolar Output Voltage with the
The addition of a second op amp to the circuit of Figure 4
can be used to generate a bipolar output voltage from a
fixed reference voltage Figure 5 This in effect gives sign
significance to the MSB of the digital input word to allow two
quadrant multiplication of the reference voltage The polarity
of the reference can also be reversed to realize the full four-
quadrant multiplication
The applied digital word is offset binary which includes a
code to output zero volts without the need of a large valued
resistor common to existing bipolar multiplying DAC circuits
Offset binary code can be derived from 2’s complement
data (most common for signed processor arithmetic) by in-
verting the state of the MSB in either software or hardware
After doing this the output then responds in accordance to
the following expression
with 1 LSB
2’s Comp
0
b
b
b
a
a
(Decimal)
1
256
512
511
256
DAC in the Current Switching Mode
e
l
V
512
REF
FIGURE 5 Providing a Bipolar Output Voltage with the DAC in the Current Switching Mode
l
0111111111
0100000000
0000000000
1111111111
1100000000
1000000000
V
2’s Comp
O
(Binary)
e
OUT1
V
REF c
now flows through the R
512
D
FIGURE 4 Providing a Unipolar Output Voltage
Digital Input
1111111111
1100000000
1000000000
0111111111
0100000000
0000000000
Applied
FB
9
True Binary
(Decimal)
Applied
where V
decimal equivalent of the 2’s complement processor data
(
applied digital input is interpreted as the decimal equivalent
of a true binary word V
V
With this configuration only the offset voltage of amplifier 1
need be nulled to preserve linearity of the DAC The offset
voltage error of the second op amp has no effect on lineari-
ty It presents a constant output voltage error and should be
nulled only if absolute accuracy is needed Another advan-
tage of this configuration is that the values of the external
resistors required do not have to match the value of the
internal DAC resistors they need only to match and temper-
ature track each other
A thin film 4 resistor network available from Beckman Instru-
ments Inc (part no 694-3-R10K-D) is ideally suited for this
application Two of the four available 10 kX resistor can be
paralleled to form R in Figure 5 and the other two can be
used separately as the resistors labeled 2R
Operation is summarized in the table below
b
1023
O
768
512
511
256
0
512
e
V
s
REF
REF
D
s

a
can be positive or negative and D is the signed
D
b
511 or 1000000000
512
512
V
REF
b
J
b
V
a
b
V
REF
1 LSB
OUT
V
V
b
REF
0
REF
REF
1 LSB
2
can be found by
2
s
V
D
OUT
s
0111111111) If the
b
l
V
b
a
REF
a
TL H 5688 – 11
a
b
l
l
V
V
0
l
1 LSB
V
V
REF
REF
s
0
l
REF
REF
a
D
1 LSB
l
l
s
l
2
2
1023