10

®

DAC7800, 01, 02

If an application requires the D/A to have zero gain error, the

circuit shown in Figure 6 may be used. Resistors R2 and R4

induce a positive gain error greater than worst-case initial

negative gain error. Trim resistors R1 and R3 provide a

variable negative gain error and have sufficient trim range to

correct for the worst-case initial positive gain error plus the

error produced by R2 and R4.

DATA INPUT

ANALOG OUTPUT

MSB

↓↓ LSB

1111 1111 1111

1000 0000 0001

1000 0000 0000

0 Volts

0111 1111 1111

0000 0000 0000

TABLE III. Bipolar Output Code.

FIGURE 5. Bipolar Configuration.

R

3

10k

1

C

10pF

DAC A

DAC B

R

10k

5

Ω

DAC780X

DGND

V

REF A

V

DD

+5V

C

D

2

R

6

Ω

20k

R

2

Ω

20k

R

10k

C

10pF

V

OUT A

V

OUT B

A1

A3

DAC7802 has a single analog common, AGND.

A1–A4, OPA602 or 1/2 OPA2107.

–

+

A2

–

+

A4

5

Ω

Ω

R

1

Ω

20k

R

4

Ω

20k

REF B

V

I

OUT B

R

FB B

–

+

AGND B

I

OUT A

R

FB A

–

+

AGND A

1µF

+

APPLICATIONS

12-BIT PLUS SIGN DACS

For a bipolar DAC with 13 bits of resolution, two solutions

are possible. As shown in Figure 7, the addition of a precision

difference amplifier and a high speed JFET switch provides

a 12-bit plus sign voltage-output DAC. When the switch

selects the op amp output, the difference amplifier serves as

a non-inverting output buffer. If the analog ground side of the

switch is selected, the output of the difference amplifier is

inverted.

Another option, shown in Figure 8, also produces a 12-bit

plus sign output without the additional switch and digital

control line.

DIGITALLY PROGRAMMABLE ACTIVE FILTER

DAC780X are shown in Figure 9 in a digitally programmable

active filter application. The design is based on the state-

variable filter, Burr-Brown UAF42, an active filter topology

that offers stable and repeatable filter characteristics.

DAC1 and DAC2 can be updated in parallel with a single word

to set the center frequency of the filter. DAC 4, which makes

use of the uncommitted op amp in UAF42, sets the Q of the

filter. DAC3 sets the gain of the filter transfer function without

changing the Q of the filter. The reverse is also true.

The center frequency is determined by f

is the ladder resistance of the D/A (typical value, 10k

Ω) and

C the internal capacitor value (1000pF) of the UAF42. Exter-

nal capacitors can be added to lower the center frequency of

the filter. But the highest center frequency for this circuit will

be about 16kHz because the effective series resistance of the

D/A cannot be less than 10k

Ω.

Note that the ladder resistance of the D/A may vary from

device to device. Thus, for best tracking, DAC2 and DAC3

should be in the same package. Some calibration may be

necessary from one filter to another.