30 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

2048

where D is the decimal equivalent of the true binary input

word This configuration inherently accepts a code (half-

external

LSB offset as needed by other bipolar multiply-

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

Applied

Decimal

Digital Input

Equivalent

a

b

MSB

LSB

1

1

1

1

1

1

1

1

1

1

1

1

4095

b

1

1

0

0

0

0

0

0

0

0

0

0

3072

b

1

0

0

0

0

0

0

0

0

0

0

0

2048

0

0

0

1

1

1

1

1

1

1

1

1

1

1

2047

b

1 LSB

a

1 LSB

0

1

0

0

0

0

0

0

0

0

0

0

1024

b

a

0

0

000000000

0

0

b

a

l

2048

01% matching

TLH5691 – 9

FIGURE 7 Obtaining a Bipolar Output from a Fixed Reference

8