Sallen Key Notch Filter

Problem:
How does a designer make an extremely high Q Sallen Key notch filter?

Solution:



The Sallen-Key circuit topology can be used to make an extremely high Q notch filter. This circuit is probably the highest Q notch filter that can be made with a single opamp. It has a lot of similarity to Twin-T filters:

• 1% components - R's AND C's - are a necessity for the deepest notch and best stability.
  
  
• The sharpest response will come when the components are matched (by using components from the same batch and by creating R3 and C3 by paralleling 2 of the values used for R1 and C1, respectively).
  
  
• It is possible to tune the center frequency over a narrow range if R3 is reduced by one or two standard resistor values and a potentiometer added in series. The depth of the notch and temperature stability will be affected, however.

R4 and R5 control both gain and Q. There is not controllability, however. R4 should never equal R5, or the circuit will become unstable. In practice, R5 should only be one or two standard values higher than R4, which will constrain the range gain to just under 6 dB. Contact Texas Instruments applications for design techniques that will allow lower values of Q with this circuit topology.