Filter Wizard - Notch Filter

Problem:
How does a designer make a Notch filter?

Solution:
Use the circuit below.



The designer is cautioned that the tuning on notch filters is extremely critical. R1, R2, R3, C1, C2, and C3 should be 1% components. In addition, R1 and R2 should be from the same batch, and R3 should be made from two of this same value and batch in parallel. Likewise, C1 and C2 should be from the same batch, and C3 made from two of this same value and batch in parallel. This will insure the deepest possible notch. The center frequency, however, may be off. If the designer wants to tune the center frequency, then R3 should be reduced in value slightly, and a small value potentiometer inserted in series. This will provide a narrow tuning range capability, at the expense of notch depth and temperature stability.

The designer, if not familiar with the topics, may want to review Coupling Capacitors (Cin in the schematic) and Virtual Grounding (Vcc/2 in the schematic).