OP490
Rev. E | Page 12 of 16
00308-024
+
–
1/4
OP490
A
+
–
1/4
OP490
B
+
–
1/4
OP490
D
+
–
1/4
OP490
C
R4
50
R8
50
R
L
7
6
5
1
4
11
2
3
13
12
9
8
10
14
R7
50
R6
5k
R3
50
R2
9k
R5
5k
R1
1k
+15V
–15V
V
IN
Figure 24. High Output Amplifier
HIGH OUTPUT AMPLIFIER
The amplifier shown in Figure 24 is capable of driving 25 V p-p
into a 1 kΩ load. Design of the amplifier is based on a bridge
configuration. A amplifies the input signal and drives the load
with the help of B. Amplifier C is a unity-gain inverter which
drives the load with help from D. Gain of the high output
amplifier with the component values shown is 10, but can easily
be changed by varying R1 or R2.
SINGLE-SUPPLY MICROPOWER QUAD
PROGRAMMABLE GAIN AMPLIFIER
The combination of a quad OP490 and the DAC8408 quad 8-bit
CMOS DAC creates a quad programmable-gain amplifier with
a quiescent supply drain of only 140 μA. 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 resistance of the DAC ladder seen by the op amp feed-
back loop. The gain of each amplifier is:
nV
V
IN
OUT
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 opens causing the op
amp output to saturate. The 10 MΩ resistors placed in parallel
with the DAC feedback loop eliminate 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.
