The circuit in Figure 1 filters 60-Hz interference from low-frequency, low-level signals. The filter exhibits 40-dB rejection (Q = 0.75) and draws 95 μA max from a single-sided 5 V supply.
Resistors R1, R2, and R3 and capacitors C1, C2, and C3 form a classic twin-T section, and IC1 and IC2 provide local and global feedback. The frequency selectivity (Q) and the rejection performance of this active filter are very sensitive to the relative matching of the capacitors and resistors in the twin-T section. Table 1 shows rejection and Q as a function of the value of RQ.
R4, R5, C4, and IC3 form a very-low-impedance reference source to bias IC1 and the twin-T section to half the supply voltage.
To configure the filter to operate at 60 Hz, choose a Q that will provide enough rejection without excessive loss of desired low-frequency signals that may be close to the filter’s notch frequency. The value of RQ is expressed as
The gain of the output amplifier is simply that of a conventional noninverting amplifier:
and the overall gain of the band-reject filter below and above the notch frequency is expressed as
If you need additional rejection, cascade filter sections. Keep in mind that you might have to modify the circuit to account for out-of-band gain multiplication.
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