REV. B

OP193/OP293/OP493

–14–

1/4 OP493

A

2

3

R1

100k

11

1

2

4

5

6

DAC A

1/4

DAC8408

DAC B

1/4

DAC8408

DAC C

1/4

DAC8408

DAC D

1/4

DAC8408

1/4 OP493

B

6

5

R2

100k

7

8

1/4 OP493

C

13

12

R3

100k

14

27

1/4 OP493

D

9

10

R4

100k

8

21

DAC DATA BUS

PINS 9(LSB)–16(MSB)

23

25

5V

1

AD589

1.23V

5V

17

18

19

20

A/B

R/

W

DS1

DS2

DAC8408ET

DIGITAL

CONTROL

SIGNALS

28

DGND

OP493

4

5V

3.6k

Figure 13. Micropower Single-Supply Quad Voltage-

Output 8-Bit DAC

A Single-Supply Micropower Quad Programmable-Gain

Amplifier

The combination of the quad OP493 and the DAC8408 quad

8-bit CMOS DAC creates a quad programmable-gain amplifier

with a quiescent supply drain of only 140

µA (Figure 14). The

digital code present at the DAC, which is easily set by a micro-

processor, determines the ratio between the fixed DAC feedback

resistor and the resistance that the DAC feedback ladder pre-

sents to the op amp feedback loop. The gain of each amplifier is:

V

Vn

OUT

IN

=

256

where n equals the decimal equivalent of the 8-bit digital code

present at the DAC.

If the digital code present at the DAC consists of all zeros, the

feedback loop will be open causing the op amp to saturate. The

10 M

Ω resistors placed in parallel with the DAC feedback loop

eliminates this problem with a very small reduction in gain

accuracy. The 2.5 V reference biases the amplifiers to the center

of the linear region providing maximum output swing.