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I think the simulation used 120k, not 120 ohms.
The first stage should have a little more gain to add some peaking so the total response of the filter is flatter.
Hi All ,
Thanks for the help . What is troubling me is the response isnt flat as simulated by "The Electritian " (what software you use). I am attaching my simulation below and draw in color BLUE with hand what is likely to be expected by the input side of the ADC .
Hi MrAl,
The filter you describe is a Bessel type with a very droopy cutoff.
We are talking about a Butterworth type with an absolutely flat response then a sharp cutoff. The active filters in a Butterworth filter are peaked a little so that the response is down only -3dB at the cutoff frequency.
It is a lowpass filter, it is DC coupled.I do have to wonder however, why you
want to choose that filter response when it sounds like he wants to
filter the dc from a low frequency noisy input mixed with dc.
I could have misunderstood the application, but it seems to be that
we would want to reject all frequencies above dc as
well as possible and not attenuate any if possible. Forcing the
response to be -3db at the cutoff doesnt seem to have any useful
purpose for this app