How does a designer create a Band Reject Filter?

Problem:
How does a designer create a Band Reject Filter?

Solution:
The best way to implement a band reject filter is to sum together the outputs of a low pass and high pass filter:



The best implementation is definitely NOT a low Q notch filter:



Some points are worth noting:

• The figure shows that although both filters have identical 3 dB points, there is much more rejection of unwanted signals in the stop band with the low pass summed with the high pass than there is with the notch filter - with the single exception of the center frequency.
  
  
• The performance increase that comes with summing low pass and high pass filter outputs comes at the expense of an additional opamp - the opamp that performs the summing function.
  
  
• Higher order low pass and high pass filters will improve the performance of the band reject filter.
  
  
• The farther apart the passbands are, the better the performance of the band reject filter.