DAC7800, 7801, 7802

10

SBAS005A

www.ti.com

If an application requires the DAC to have zero gain error, the

a positive gain error greater than worst-case initial negative

negative gain error and have sufficient trim range to correct

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

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. The addition of a precision difference amplifier and

a high speed JFET switch provides a 12-bit plus sign voltage-

output DAC, see Figure 7. When the switch selects the op

amp output, the difference amplifier serves as a noninverting

output buffer. If the analog ground side of the switch is

selected, the output of the difference amplifier is inverted.

Another option, see Figure 8, also produces a 12-bit plus sign

output without the additional switch and digital control line.

DIGITALLY PROGRAMMABLE ACTIVE FILTER

See Figure 9 for the DAC780x in a digitally programmable

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

variable filter, Texas Instruments UAF42, an active filter topol-

ogy 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 ladder resistance of the DAC (typical value, 10k

Ω) and C

the internal capacitor value (1000pF) of the UAF42. External

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

DAC cannot be less than 10k

Ω.

Note that the ladder resistance of the DAC 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.