AD825 データシートの表示（PDF） - Analog Devices

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AD825

Low Cost, General-Purpose High Speed JFET Amplifier

Analog Devices 

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.

C1 =

2π

1.414

fCUTOFF R1

C2

( farads) =

2π

0.707

fCUTOFF R1

( ) R1 = R2 = User Selected Typically 10 kΩ to 100 kΩ

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

better than 40 dB of high frequency rejection is provided.

AD825

C1

24pF

R1

R2

9.31kΩ 9.31kΩ

VIN

C2

6pF

+5V

C3

0.1µF

AD825

VOUT

C4

0.1µF

–5V

Figure 33. Second-Order Butterworth Low-Pass Filter

0

–10

–20

–30

–40

–50

–60

–70

–80

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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