OP471 Datasheet, PDF (12/16 Pages) Analog Devices – High Speed, Low Noise Quad Operational Amplifier

OP471
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_{OUT = –} 256
V_IN
n
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 MW resistors placed in parallel
with the DAC feedback loop eliminates this problem with a very
small reduction in gain accuracy.
V_INA
V_INB
DAC-8408ET
R_FBA
R_FBB
V_DD
V_REFA
DAC A
I_OUT1A
I_OUT2A/2B
V_REFB
DAC B
I_OUT1B
V_INC
R_FBC
V_IND
R_FBD
DAC DATA BUS
PINS 9 (LSB) – 16 (MSB)
V_REFC
DAC C
I_OUT1C
I_OUT2C/2D
V_REFD
DAC D
I_OUT1D
DGND
R1
20M⍀
+15V
1/4
OP470E
–15V
R2
20M⍀
1/4
OP470E
R3
20M⍀
1/4
OP470E
R4
20M⍀
1/4
OP470E
Figure 17. Quad Programmable Gain Amplifier
V_OUTA
V_OUTB
V_OUTC
V_OUTD
–12–
REV. A