Datasheet

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.
R1f
C1
CUTOFF
π
=
2
414.1
()
R1f
faradsC2
CUTOFF
π
=
2
707.0
(
)
Ωk100Ωk10 toTypicallySelectedUserR2R1 ==
A plot of the filter frequency response is shown in Figure 34;
better than 40 dB of high frequency rejection is provided.
AD825
C3
0.1µF
C4
0.1µF
V
OUT
V
IN
C2
6pF
–5V
C1
24pF
R1
9.31kΩ
R2
9.31kΩ
+5V
00876-E-031
Figure 33. Second-Order Butterworth Low-Pass Filter
FREQUENCY (Hz)
10k 100M100k
HIGH FREQUENCY REJECTION (dB)
1M 10M
0
–10
–20
–30
–40
–50
–60
–70
–80
00876-E-032
Figure 34. Frequency Response of Second-Order Butterworth Filter