Careful PC board ground layout techniques should be

used to minimize crosstalk between DAC outputs, the

reference input(s), and the digital inputs. This is partic-

ularly important if the reference is driven from an AC

source. Figure 7 shows suggested PC board layouts for

minimizing crosstalk.

Unipolar Output

In unipolar operation, the output voltages and the refer-

ence input(s) are the same polarity. The unipolar circuit

configuration is shown in Figure 8 for the MAX500. The

device can be operated from a single supply with a

slight increase in zero error (see

Output Buffer

Amplifiers section). To avoid parasitic device turn-on,

respect to AGND. The unipolar code table is given in

Table 3.

Bipolar Output

Each DAC output may be configured for bipolar opera-

tion using the circuit in Figure 9. One op amp and two

resistors are required per channel. With R1 = R2:

word in Register A.

Table 4 shows the digital code versus output voltage

for the circuit in Figure 9.

Offsetting AGND

AGND can be biased above DGND to provide an arbi-

trary nonzero output voltage for a “zero” input code. This

input word. Since AGND is common to all four DACs,

Since AGND current is a function of the four DAC

codes, it should be driven by a low-impedance source.

CMOS, Quad, Serial-Interface

8-Bit DAC

10

______________________________________________________________________________________

DAC A

MAX500

4

14

AGND

2

DGND

3

6

+

-

+

-

5

+15V

-5V (OR GND)

DIGITAL INPUTS NOT SHOWN

Figure 10. AGND Bias Circuit

Table 3. Unipolar Code Table

Table 4. Bipolar Code Table

1 0 0 0

DAC CONTENTS

0 0 0 1

MSB

LSB

ANALOG

OUTPUT

1 1 1 1

1 1 1 1

127

128

1

128

1 0 0 0

0 0 0 0

0V

0 1 1 1

1 1 1 1

1

128

0 0 0 0

0 0 0 0

128

128

0 0 0 0

0 0 0 1

127

128

1 0 0 0

DAC CONTENTS

0 0 0 1

MSB

LSB

ANALOG

OUTPUT

1 1 1 1

1 1 1 1

255

256

129

256

1 0 0 0

0 0 0 0

128

256

2

0 1 1 1

1 1 1 1

127

256

0 0 0 0

0 0 0 0

0V

0 0 0 0

0 0 0 1

1

256

1

256

1

256