AD825

Rev. F | Page 11 of 12

SECOND-ORDER LOW-PASS FILTER

A second-order Butterworth low-pass filter can be implemented

using the AD825 as shown in Figure 33. The extremely low bias

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

consequently, small capacitor values without concern for

developing 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.

R1

f

C1

CUTOFF

π

=

2

414

.

1

()

R1

f

farads

C2

CUTOFF

π

=

2

707

.

0

(

)

Ω

k

100

Ω

k

10

to

Typically

Selected

User

R2

R1

=

=

A plot of the filter frequency response is shown in Figure 34;

better than 40 dB of high frequency rejection is provided.

AD825

C3

0.1

µF

C4

0.1

µF

C2

6pF

–5V

C1

24pF

R1

9.31k

Ω

R2

9.31k

Ω

+5V

Figure 33. Second-Order Butterworth Low-Pass Filter

FREQUENCY (Hz)

10k

100M

100k

1M

10M

0

–10

–20

–30

–40

–50

–60

–70

–80

Figure 34. Frequency Response of Second-Order Butterworth Filter