Datasheet

AD823
Rev. A | Page 17 of 20
Second-Order Low-Pass Filter
Figure 43 depicts the AD823 configured as a second-order
Butterworth low-pass filter. With the values as shown, the
corner frequency will be 200 kHz. The equations for component
selection are shown below.
R1 = R2 = User Selected (Typical Values: 10 kΩ to 100 kΩ)
()
R1f
C2
Rf
.
faradsC1
cutoffcutoff
2π
707.0
;
12π
4141
==
1/2
AD823
C3
0.1µF
+5V
C4
0.1µF
V
OUT
V
IN
C1
28pF
–5V
C2
56pF
R1
20kΩ
R2
20kΩ
50pF
00901-A-042
Figure 43. Second-Order Low-Pass Filter
A plot of the filter is shown in Figure 44; better than 50 dB of
high frequency rejection is provided.
FREQUENCY (Hz)
–40
HIGH FREQUENCY REJECTION (dB)
1k
–50
–30
–60
0
–20
10k 100k
1M
10M 100M
–10
V
DB
– V
OUT
00901-A-043
Figure 44. Frequency Response of Filter
Single-Supply Half-Wave and Full-Wave Rectifiers
An AD823 configured as a unity-gain follower and operated
with a single supply can be used as a simple half-wave rectifier.
The AD823’s inputs maintain pico-amp level input currents
even when driven well below the minus supply. The rectifier
puts that behavior to good use, maintaining an input impedance
of over 10
11
Ω for input voltages from 1 V from the positive
supply to 20 V below the negative supply.
The full-wave and half-wave rectifier shown in Figure 46 oper-
ates as follows: when V
IN
is above ground, R1 is bootstrapped
through the unity-gain follower A1 and the loop of Amplifier
A2. This forces the inputs of A2 to be equal, thus no current
flows through R1 or R2, and the circuit output tracks the input.
When V
IN
is below ground, the output of A1 is forced to ground.
The noninverting input of Amplifier A2 sees the ground level
output of A1; therefore, A2 operates as a unity-gain inverter. The
output at Node C is then a full-wave rectified version of the
input. Node B is a buffered half-wave rectified version of the
input. Input voltage supply to ±18 V can be rectified, depending
on the voltage supply used.
3
2
1
8
4
0.01µF
V
IN
+V
S
1/2
AD823
HALF-WAVE
RECTIFIED OUTPUT
FULL-WAVE
RECTIFIED OUTPUT
A1
R2
100kΩ
R1
100kΩ
6
5
7
1/2
AD823
A2
A
B
C
00901-A-044
A2
Figure 45.
100
A
B
C
2V
2V 200µs
2V
90
10
0%
00901-A-045
Figure 46. Single-Supply Half-Wave and Full-Wave Rectifier