DAC0830LCM datasheet(13/28 Pages) NSC

13 / 28 page

DAC0830 Series Application Hints

(Continued)

Always use the internal R

fb 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 (I

OUT1).

2.3 Op Amp Considerations

The op amp used in

Figure 7 should have offset voltage

nulling capability (See Section 2.5).

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-

cations. BI-FET™ op amps are highly recommended for use

with these DACs because of their very low input current.

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, R

fb, 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, C

C 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 V

REF 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.

2.4 Bipolar Output Voltage with a Fixed Reference

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

multiplication: ±V

REFx±Digital Code= ±VOUT. This circuit is

shown in

Figure 9.

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 k

Ω resistors can be paralleled to form R in

Figure 9 and the other two can be used independently as the

resistances labeled 2R.

00560837

FIGURE 6.

00560838

FIGURE 7.

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