DAC1006/1007/1008 — Simple Hookup for a “Quick Look”
(Continued)
5.1.2 Providing a Bipolar Output Voltage with the DAC
in the Current Switching Mode
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 signifi-
cance to the MSB of the digital input word to allow two quad-
rant 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 inverting
the state of the MSB in either software or hardware. After do-
ing this the output then responds in accordance to the follow-
ing expression:
where V
REF
can be positive or negative and D is the signed
decimal equivalent of the 2’s complement processor data.
(−512
≤
D
≤
+511 or 1000000000
≤
D
≤
0111111111). If the ap-
plied digital input is interpreted as the decimal equivalent of
a true binary word, V
OUT
can be found by:
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 linearity.
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 re-
sistors required do not have to match the value of the inter-
nal DAC resistors; they need only to match and temperature
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 k
Ω
resistor can be
paralleled to form R in
Figure 5 and the other two can be
used separately as the resistors labeled 2R.
DIGITAL INPUT CODE
DS005688-39
FIGURE 2. Current Mode Switching
DS005688-40
OP
AMP
C
C
pF
R
j
ts µS
LF356
22
∞
3
LF351
24
∞
4
LF357
10
2.4k
1.5
FIGURE 3. Converting I
OUT
to V
OUT
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PrintDate=1998/11/17 PrintTime=11:38:09 46711 ds005688 Rev. No. 4
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