DAC0830, DAC0832

SNAS534B – MAY 1999 – REVISED MARCH 2013

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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

This configuration features several improvements over existing 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 resistors around the second amplifier

do not have to match the internal DAC resistors, they need only to match and temperature 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 20 and the other two

can be used independently as the resistances labeled 2R.

Zero Adjustment

For accurate conversions, the input offset voltage of the output amplifier must always be nulled. Amplifier offset

errors create an overall degradation of DAC linearity.

Figure 19.

OP Amp

(O to Full Scale)

LF356

22 pF

4

μs

LF351

22 pF

5

μs

10 pF

2

μs

(1)

2.4 k

Ω RESISTOR ADDED FROM−INPUT TO GROUND TO

INSURE STABILITY

*THESE RESISTORS ARE AVAILABLE FROM BECKMAN INSTRUMENTS, INC. AS THEIR PART NO. 694-3-R10K-

D

Figure 20.

14

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