REV. A

OP471

–12–

Quad Programmable Gain Amplifier

The combination of the quad OP471 and the DAC8408, a quad

8-bit CMOS DAC, creates a space-saving quad programmable gain

amplifier. The digital code present at the DAC, which is easily

set by a microprocessor, determines the ratio between the fixed

DAC feedback resistor and the impedance the DAC ladder presents

to the op amp feedback loop. Gain of each amplifier is:

V

V

= –

256

n

OUT

IN

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 output to saturate. The 20 M

W resistors placed in parallel

with the DAC feedback loop eliminates this problem with a very

small reduction in gain accuracy.

1/4

OP470E

+15V

–15V

DAC A

DGND

DAC-8408ET

DAC DATA BUS

PINS 9 (LSB) – 16 (MSB)

R1

20M

1/4

OP470E

DAC B

R2

20M

1/4

OP470E

DAC C

R3

20M

1/4

OP470E

R4

20M

DAC D

A

B

C

D

Figure 17. Quad Programmable Gain Amplifier