BANDPASS SELECTION FOR BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 68

R1

R3

R R

C2
2C

OUTPUT

INPUT

R2
0.5R

C1

C3

C

C

Figure 2.18 The notch frequency of the simple twin-T filter is f notch 1/2π(R1)(C1) if C1 C3, C2 2C1, R1 R3, and R2 R1/2.
However, the use of precise and tightly matched components is extremely important to yield a deep notch at the required frequency.

Figure 2.19 Passive notch filters can be built into small enclosures with BNC connectors on each
end and placed between equipment stages.

the output load. The circuit of Figure 2.20 shows how a unity-gain follower bootstraps the
network in an active twin-T notch filter. Since output of the op-amp presents a very low

impedance, the notch frequency and depth are not changed. However, the Q value of the
fi

fi

filter increases proportionately to the level of signal that is fed back to the junction of R2
and C2 (the point that is grounded in a passive twin-T notch fi filter).

filter power line interference. The power
line frequency in many countries deviates quite a bit from the nominal 50 or 60 Hz. A sec-
ond op-amp can be added as shown in Figure 2.21 to control the Q value of the

A very high Q value is not always desirable to fi

filter. Here,
the amount of feedback that is provided to the R2/C2 junction is set by potentiometer R4.
An op-amp is needed to bu

fi

er the feedback signal to ensure a constant low impedance at
the R2/C2 junction so that the notch frequency and depth do not change as a function of
the potentiometer setting.

ff