AD712

REV. B

–13–

SECOND ORDER LOW PASS FILTER

Figure 38 depicts the AD712 configured as a second order

Butterworth low pass filter. With the values as shown, the corner

frequency will be 20 kHz; however, the wide bandwidth of the

AD712 permits a corner frequency as high as several hundred

kilohertz. Equations for component selection are shown below.

R1 = R2 = user selected (typical values: 10 k

Ω – 100 kΩ)

C1(in farads )

=

1.414

(2

π)( f

C2

=

0.707

(2

π)( f

+15V

1/2

AD712

0.1 F

0.1 F

–15V

C2

280pF

R2

20k

R1

20k

C1

560pF

Figure 38. Second Order Low Pass Filter

An important property of filters is their out-of-band rejection.

The simple 20 kHz low pass filter shown in Figure 38, might be

used to condition a signal contaminated with clock pulses or

sampling glitches which have considerable energy content at

high frequencies.

The low output impedance and high bandwidth of the AD712

minimize high frequency feedthrough as shown in Figure 39.

The upper trace is that of another low-cost BiFET op amp

showing 17 dB more feedthrough at 5 MHz.

REF 20.0 dBm

10 dB/DIV

RANGE 15.0 dBm

OFFSET .0 Hz

0 dB

CENTER 5 000 000.0 Hz

RBW 30 kHz

SPAN 10 000 000.0 Hz

ST .8 SEC

VBW 30 kHz

AD712

TYPICAL BIFET

Figure 39.