shown in

Figure 4 where the sign of the reference voltage

has been changed to provide a positive output voltage Note

pin

512 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

significance to the MSB of the digital input word to allow two

quadrant 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 in-

verting the state of the MSB in either software or hardware

After doing this the output then responds in accordance to

the following expression

D

512

decimal equivalent of the 2’s complement processor data

applied digital input is interpreted as the decimal equivalent

512

J

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 lineari-

ty 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

resistors required do not have to match the value of the

internal DAC resistors they need only to match and temper-

ature 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 kX resistor can be

paralleled to form R in

Figure 5 and the other two can be

used separately as the resistors labeled 2R

Operation is summarized in the table below

Applied

2’s Comp

2’s Comp

Applied

True Binary

(Decimal)

(Binary)

Digital Input

(Decimal)

a

b

a

511

0111111111

1111111111

1023

b

a

256

0100000000

1100000000

768

b

0

0000000000

1000000000

512

0

0

b

1

1111111111

0111111111

511

b

1 LSB

a

1 LSB

b

256

1100000000

0100000000

256

b

a

b

512

1000000000

0000000000

0

b

a

l

512

FIGURE 4 Providing a Unipolar Output Voltage

TLH5688 – 11

FIGURE 5 Providing a Bipolar Output Voltage with the DAC in the Current Switching Mode

9