AD823
Second-Order Low-Pass Filter
Single-Supply Half-Wave and Full-Wave Rectifiers
Figure 43 depicts the AD823 configured as a second-order
Butterworth low-pass filter. With the values as shown, the
corner frequency equals 200 kHz. Component selection is
shown in the following equations:
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 inputs maintain picoamp 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 1011 Ω for input voltages from within 1 V of the positive
supply to 20 V below the negative supply.
R1 = R2 = User Selected (Typical Values: 10 kΩ to 100 kΩ)
1.414
2πfcutoff × R1
C1
(
farads =
)
The full-wave and half-wave rectifier shown in Figure 4ꢀ
operates as follows: when VIN is above ground, R1 is boot-
strapped 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 VIN 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.
0.707
2πfcutoff × R1
C2 =
+5V
1/2
C2
56pF
C3
0.1µF
R1
R2
20kΩ
20kΩ
V
IN
C1
28pF
V
AD823
OUT
50pF
R1
100kΩ
R2
100kΩ
C4
0.1µF
–5V
+V
8
S
0.01µF
Figure 43. Second-Order Low-Pass Filter
6
5
A
7
3
2
C
A plot of the filter is shown in Figure 44; better than ꢀ0 dB of
high frequency rejection is provided.
A2
A2
1
V
FULL-WAVE
RECTIFIED OUTPUT
A1
1/2
AD823
IN
1/2
0
4 AD823
B
–10
HALF-WAVE
RECTIFIED OUTPUT
V
– V
OUT
DB
–20
–30
–40
Figure 45. Full-Wave and Half-Wave Rectifier
2V
200µs
–50
–60
100
90
A
1k
10k
100k
FREQUENCY (Hz)
10M
100M
1M
Figure 44. Frequency Response of Filter
B
C
10
0%
2V
Figure 46. Single-Supply Half-Wave and Full-Wave Rectifier
Rev. B | Page 17 of 20
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