TWIN_T_NOITCH_FILTER

Published:2009/7/8 22:09:00 Author:May | From:SeekIC

This 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. Commercial grade components with 5%-1070 tolerance produce a null depth of at least 30 to 40 dB. When a twin-T network is combined with a TL081 op amp in a circuit, an active filter can be implemented. The added resistor capacitor network, R2 and C2, work 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: R2 C1 Q = — = — 1R2 C2For a 60-Hz notch filter with a @ of 5, it is usually best to pick the C1 capacitor value and calculate the resistor R1. Let C1 = 0.22 μF. Then: R1 = 12 KΩ R1 = 120 KΩ C2 = 0.047/μF Standard 5% resistors and 10% capacitors produce a notch depth of about 40 dB, as shown in the frequency response curve.



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