Data Sheet

AD823

Rev. E | 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 equals 200 kHz. Component selection is

shown in the following equations:

R1 = R2 = User Selected (Typical Values: 10 kΩ to 100 kΩ)



R1

f

.

farads

C1



2π

414

1

R1

f

C2



2π

707

.

0

1/2

AD823

C3

0.1µF

+5V

C4

0.1µF

C1

28pF

–5V

C2

56pF

R1

20kΩ

R2

20kΩ

50pF

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

1k

–50

–30

–60

0

–20

10k

100k

1M

10M

100M

–10

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 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

supply to 20 V below the negative supply.

The full-wave and half-wave rectifier shown in Figure 45

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

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

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

A2

Figure 45. Full-Wave and Half-Wave Rectifier

100

A

B

C

2V

200µs

2V

10

0%

90

Figure 46. Single-Supply Half-Wave and Full-Wave Rectifier