A notch filter is used to reject or block a frequency or band of frequencies. These filters are often designed into audio and instrumentation systems to eliminate a single frequency, such as 60 Hz. Perhaps the best-known passive notch filter is the “twin-T'” filter. The circuit is shown in Figure 3-23.
If the six components are carefully matched, theoretically you can obtain an almost infinite rejection at the null frequency. Commercial grade components (5%-10% tolerance) produce a null depth of at least 30 to 40 dB
When this twin-T network is combined with a TL081 op amp in a circuit, an active filter can be implemented as shown in Figure 3-24. Notice the added resistor capacitor
network (R2, C2), effectively in parallel with the original twin- T network, on the input of the filter. These networks set the Q of the filter. The op amp is basically connected as a unity-gain voltage follower. The Q is found from:
Next calculate R2 in the Q network.
Finally, calculate C2 from the equation.
Standard 5% resistors and 10% capacitors produce a notch depth of about 40 dB as shown in the frequency response curve (Figure 3-25).