DAC1219 NSC [National Semiconductor], DAC1219 Datasheet - Page 8

• Manufacturer Part Number: DAC1219
• Description: 12-Bit Binary Multiplying D/A Converter
• Manufacturer: NSC [National Semiconductor]

MSB
Application Hints
3 0 OBTAINING A BIPOLAR OUTPUT VOLTAGE
FROM A FIXED REFERENCE
The addition of a second op amp to the circuit of Figure 2
can generate a bipolar output voltage from a fixed reference
voltage ( Figure 7 ) This in effect gives sign significance to
the MSB of the digital input word to allow two quadrant mul-
tiplication of the reference voltage The polarity of the refer-
ence voltage can also be reversed to realize full 4-quadrant
multiplication
The output responds in accordance to the following expres-
sion
Where 1 LSB
1
1
1
0
0
0
0 1% matching
1
1
0
1
1
0
V
O
e
e
1
0
0
1
0
0
V
l
V
2048
REF
REF
1
0
0
1
0
0
l
D
1
0
0
1
0
0
b
2048
2048
Digital Input
(Continued)
FIGURE 7 Obtaining a Bipolar Output from a Fixed Reference
Applied
1
0
0
1
0
0
0
1
0
0
1
0
0
s
D
s
1
0
0
1
0
0
4095
1
0
0
1
0
0
1
0
0
1
0
0
1
0
0
1
0
0
8
where D is the decimal equivalent of the true binary input
word This configuration inherently accepts a code (half-
scale or D
external
ing DAC circuits
Only the offset voltage of amplifier A1 need be nulled to
preserve linearity The gain setting resistors around A2 must
match and track each other A thin film 4-resistor network
available from Beckman Instruments Inc (part no 694-3-
R10K-D) is ideally suited for this application Two of the four
resistors can be paralleled to form R and the other two can
be used separately as the resistors labeled 2R
Operation is summarized in the table below
LSB
1
0
0
1
0
0
Equivalent
Decimal
e
LSB offset as needed by other bipolar multiply-
4095
3072
2048
2047
1024
2048) to provide 0V out without requiring an
0
V
REF
b
b
V
a
b
V
REF
1 LSB
V
b
V
REF
0
REF
REF
1 LSB
2
2
V
OUT
b
V
b
a
REF
a
a
b
l
l
V
V
TL H 5691 – 9
l
1 LSB
V
V
REF
REF
0
l
REF
REF
a
1 LSB
l
l
l
2
2