Electronic – Notch filter with 4 Hz Bandwidth

active-filterfilteroperational-amplifier

I want to design a notch filter for eliminating 100Hz waveform. I am using the below schematic. I tried the circuit with bandwidth 300Hz but this will affect the frequency of 20Hz to 200Hz signal also. As well I am getting the -13dB at 100Hz( Vout(peak-peak) = 200mV and Vin(peak-peak) = 1V ) but I want the output signal as low as 10mV.

For reducing the effect of the filter on other frequency I decrease filter bandwidth(4Hz) by using below calculator and then change the R4 and R5 resistor value(mention in the schematic). But it is now giving the worst result. Now it is giving the output of -2.5dB(750mV(peak-peak)) at 100Hz. Or I can say there is negligible effect at 100hZ or other frequency.

Need to ask how to improve this circuit for deeper notch and approx (peak-peak) 10mV(or less) signal at the output for 100Hz frequency.
Thank you in advance.enter image description here

Best Answer

A Design Spec for any Active Filter must include many variables.

  • Gain PassBand ( I used 3dB )
  • PassBand Ripple
  • PassBand BW ( not defined here ) and relates with PassBand ripple or gain error
  • Attenuation BandStop ( 1/20 = -26 dB needed at 100Hz)
  • BW Bandstop ( not defined here which relates to 100 Hz signal f error. (I used 4Hz)

Some variations in spec desciptions may exist, but must define all important limits needed.

My specs

Pass Band gain: 1.5 ( 3.522 dB ) PB Ripple max: 1dB
center frequency : 100 Hz
Corner frequency attenuation 0.5 dB
Stages : 2 4th order
Input DC offset; minimal

enter image description here

This exceeds the requirements by a wide margin. and can be further optimized by offseting each notch e.g. +/-2 Hz if a wider/deeper f bandstop depth is needed.

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