AD825
+5V
SECOND-ORDER LOW-PASS FILTER
C1
24pF
C3
0.1µF
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.
R1
R2
9.31kΩ 9.31kΩ
V
IN
C2
6pF
V
AD825
OUT
C4
0.1µF
–5V
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.
Figure 33. Second-Order Butterworth Low-Pass Filter
0
–10
–20
–30
–40
–50
–60
–70
–80
1.414
C1 =
2π fCUTOFF R1
0.707
2π fCUTOFF R1
C2
(
farads =
)
R1 = R2 =User Selected Typically 10 kΩ to100 kΩ
A plot of the filter frequency response is shown in Figure 34;
better than 40 dB of high frequency rejection is provided.
10k
100k
1M
10M
100M
FREQUENCY (Hz)
Figure 34. Frequency Response of Second-Order Butterworth Filter
Rev. F | Page 11 of 12
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