Bipolar Output

Using Figure 4’s circuit, the MAX512/MAX13 can be con-

figured for bipolar outputs. Table 4 lists the bipolar codes

and corresponding output voltages. There are two ways to

achieve

rail-to-rail

outputs:

1)

Operate

the

MAX512/MAX513 with a single supply and positive refer-

ence voltages or 2) Use dual supplies with a positive or

negative voltage at REFAB and a positive voltage at REFC.

In either case, the op amps need dual supplies. When

using the dual-supply mode, possible errors associated

with the divide-by-two attenuator and gain-of-two buffer are

eliminated (see the

Output Buffer Amplifier section). For

maximum output swing of all outputs in dual-supply mode,

With dual supplies, DACs A and B can perform four-

quadrant multiplication. Please note that in dual-supply

1.5V). Because REFC accepts only positive inputs,

DAC C performs two-quadrant multiplication.

Figure 4 shows Maxim’s ICL7612A with rail-to-rail input

common-mode range and rail-to-rail output voltage

swing—ideal for a high output voltage swing from low

supply voltages.

RF Applications

Both the MAX512 and MAX513 can bias GaAs FETs,

where the gate of the FETs must be negatively biased

to ensure that there is no drain current. In a typical

application, power to the RF amplifiers should not be

turned on until the bias voltages provided by DAC A

and DAC B are fully established; likewise, the supply

should be turned off before the bias voltage is switched

off. Figure 5 shows how DAC B supplies the negative

The DAC A and DAC B outputs are also ideal for con-

trolling VCOs in mobile radios or cellular phones. Other

applications include varactor and PIN diode circuits.

The unbuffered DAC C provides a span within GND

cally adjusts offset and gain in the system.

1 [John Wachsmann. “A High-Efficiency GaAs MMIC Power Amplifier for

1.9GHz PCS Applications,” Proceedings of the First Annual Wireless

Symposium, pp. 375, Penton Publishing, Jan. 1993.]

Low-Cost, Triple, 8-Bit Voltage-Output DACs

with Serial Interface

______________________________________________________________________________________

13

DAC CONTENTS

B7 B6 B5 B4 B3 B2 B1 B0

ANALOG

OUTPUT

1

1

1

1

1

1

1

1

+×









REF_

255

256

1

0

0

0

0

0

0

1

+×









REF_

129

256

1

0

0

0

0

0

0

0

+×









=+

REF_

128

256

REF_

2

0

1

1

1

1

1

1

1

+×









REF_

127

256

0

0

0

0

0

0

0

1

+×









REF_

1

256

0

0

0

0

0

0

0

0

0V

Note :

1LSB

REF_

2

REF_

1

256

ANALOG OUTPUT

REF_

D

256

8

=×

=

×









=×









−

Table 3. Unipolar Code Table

DAC CONTENTS

B7 B6 B5 B4 B3 B2 B1 B0

ANALOG

OUTPUT

1

1

1

1

1

1

1

1

+×









REF_

127

128

1

0

0

0

0

0

0

1

+×









REF_

1

128

0

1

1

1

1

1

1

1

−×









REF_

1

128

0

0

0

0

0

0

0

1

−×









REF_

127

128

0

0

0

0

0

0

0

0

−×









−

REF_

128

128

=

REF_

1

0

0

0

0

0

0

0

0V

Note :

1LSB

REF_

2

REF_

1

128

ANALOG OUTPUT

REF_

D

128

1

8 - 1)

=×

=

×









=×

−









−(

Table 4. Bipolar Code Table