DAC0830 Series Application Hints
(Continued)
Always use the internal Rfb resistor to create an output
voltage since this resistor matches (and tracks with tempera-
ture) the value of the resistors used to generate the output
current (IOUT1).
00560837
FIGURE 6.
00560838
FIGURE 7.
2.3 Op Amp Considerations
2.4 Bipolar Output Voltage with a Fixed Reference
The op amp used in Figure 7 should have offset voltage
nulling capability (See Section 2.5).
The addition of a second op amp to the previous circuitry can
be used to generate a bipolar output voltage from a fixed
reference voltage. This, in effect, gives sign significance to
the MSB of the digital input word and allows two-quadrant
multiplication of the reference voltage. The polarity of the
reference can also be reversed to realize full 4-quadrant
The selected op amp should have as low a value of input
bias current as possible. The product of the bias current
times the feedback resistance creates an output voltage
error which can be significant in low reference voltage appli-
±
±
±
multiplication: VREFx Digital Code= VOUT. This circuit is
shown in Figure 9.
™
cations. BI-FET op amps are highly recommended for use
with these DACs because of their very low input current.
This configuration features several improvements over exist-
ing circuits for bipolar outputs with other multiplying DACs.
Only the offset voltage of amplifier 1 has to be nulled to
preserve linearity of the DAC. The offset voltage error of the
second op amp (although a constant output voltage error)
has no effect on linearity. It should be nulled only if absolute
output accuracy is required. Finally, the values of the resis-
tors around the second amplifier do not have to match the
internal DAC resistors, they need only to match and tem-
perature 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. These
resistors are matched to 0.1% and exhibit only 5 ppm/˚C
resistance tracking temperature coefficient. Two of the four
available 10 kΩ resistors can be paralleled to form R in
Figure 9 and the other two can be used independently as the
resistances labeled 2R.
Transient response and settling time of the op amp are
important in fast data throughput applications. The largest
stability problem is the feedback pole created by the feed-
back resistance, Rfb, and the output capacitance of the DAC.
This appears from the op amp output to the (−) input and
includes the stray capacitance at this node. Addition of a
lead capacitance, CC in Figure 8, greatly reduces overshoot
and ringing at the output for a step change in DAC output
current.
Finally, the output voltage swing of the amplifier must be
greater than VREF to allow reaching the full scale output
voltage. Depending on the loading on the output of the
±
amplifier and the available op amp supply voltages (only 12
volts in many development systems), a reference voltage
less than 10 volts may be necessary to obtain the full analog
output voltage range.
13
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