MF6 Datasheet, PDF(16/20 Page) National Semiconductor (TI) – 6th Order Switched Capacitor Butterworth Lowpass

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MF6 Datasheet, PDF (16/20 Pages) National Semiconductor (TI) – 6th Order Switched Capacitor Butterworth Lowpass

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2.0 Designing with the MF6 (Continued)

2.3 IMPLEMENTING A âNOTCHâ FILTER WITH THE MF6

A ânotchâ filter with 60 dB of attenuation can be obtained by

using one of the Op-Amps, available in the MF6, and three

external resistors. The circuit and amplitude response are

shown in Figures 15, 16.

The frequency where the ânotchâ will occur is equal to the

frequency at which the output signal of the MF6 will have the

same magnitude but be 180 degrees out of phase with its in-

put signal. For a sixth order Butterworth filter 180Ë phase

shift occurs where f = fn = 0.742 fc. The attenuation at this

frequency is 0.12 dB which must be compensated for by

making R1 = 1.014 x R2.

Since R1 does not equal R2 there will be a gain inequality

above and below the notch frequency. At frequencies below

the notch frequency (f << fn), the signal through the filter

has a gain of one and is non-inverting. Summing this with the

input signal through the Op-Amp yields an overall gain of two

or +6 dB. For f >> fn, the signal at the output of the filter is

greatly attenuated thus only the input signal will appear at

the output of the Op-Amp. With R3 = R1 = 1.014 R2 the over-

all gain is 0.986 or â0.12 dB at frequencies above the notch.

FIGURE 12. Cascading Two MF6s

DS005065-25

DS005065-26

FIGURE 13. One MF6-50 vs. Two MF6-50s Cascaded

DS005065-27

FIGURE 14. Phase Response of

Two Cascaded MF6-50s

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