AD825

–10–

REV. C

Second Order Low-Pass Filter

A second order Butterworth low-pass filter can be implemented

using the AD825 as shown in Figure 28. The extremely low bias

currents of the AD825 allow the use of large resistor values, and

consequently small capacitor values, without concern for devel-

oping large offset errors. Low current noise is another factor in

permitting the use of large resistors without having to worry

about the resultant voltage noise.

With the values shown, the corner frequency will be 1 MHz.

The equations for component selection are shown below. Note

that the noninverting input (and the inverting input) has an

input capacitance of 6 pF. As a result, the calculated value of

C1 (12 pF) is reduced to 6 pF.

C1

=

1.414

2

π f

C2( farads)

=

0.707

2

π f

R1

= R2 = user selected typically10kΩ to100 kΩ

()

A plot of the filter frequency response is shown in Figure 29;

better than 40 dB of high frequency rejection is provided.

AD825

C3

0.1 F

+5V

C4

0.1 F

C2

6pF

–5V

C1

24pF

R1

9.31k

R2

9.31k

Figure 28. Second Order Butterworth Low-Pass Filter

FREQUENCY – Hz

10k

100M

100k

1M

10M

0

–10

–20

–30

–40

–50

–60

–70

–80

Figure 29. Frequency Response of Second Order

Butterworth Filter