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2nd order Lowpass filter help

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D.M.

New Member
I am designing a sensor to measure the biopotential from a person (EKG)

If i am trying to design a 2nd order lowpass filter with a corner frequency of 250 Hz would using a cascaded RC filter with the output voltage going into a voltage follower (lm741) be a decent approach. i have attached a picture of the circuit i am describing
9201-untitled.jpg
. Will there be an issue with the capacitor connected to the + terminal of the op-amp knowing that certain frequecies will short the cap.
The output from the voltage follower will be connected to the AD202K isolation transformer in order to isolate the person under measurement from any harmful surges of current.

Just wondering if my filter choice will work under most cicrcumstances. I am new to the design realm in electronics so any help will be nice.

Mythoughts:

First, each RC branch's corner frequency is 250Hz yet when they are cascaded the corner frequency is 150 Hz. I am only saying this because when i simulate a BODE plot, or even plug in the values in my transfer function at 150Hz i get attenuation by a factor of .707 (-3dB). My understanding of the corner frequency is the frequency in which the circuit has half power or 20log(1/sqrt(2)). When calculated or simulated at 250Hz i get attenuation of 0.5.

Does the second branch really change the cornerfrequency?

I know the second branch of the RC section (R2 and C2) will load the first part, which is why i increased R2 by a factor of 10.

Second:

The cap is only a short a high frequencies and considering my corner freq is 250should i forsee a problem?

are there other issues i should focus on?




or should i use a butterworth filter?
 
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audioguru

Well-Known Member
Most Helpful Member
You have a horrible droopy filter.
Why not make a Sallen and Key Butterworth active lowpass filter? It is -3db at the cutoff frequency and has a sharp attenuation of higher frequencies.
 

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D.M.

New Member
thank you for replying.

what do u mean by droopy? i figured this filter is crap, but i had to start somewhere.
 

MikeMl

Well-Known Member
Most Helpful Member
thank you for replying.

what do u mean by droopy? i figured this filter is crap, but i had to start somewhere.

Look at the red and blue traces. Note that your two poles stack up and cause the attenuation at 250 Hz to be -6db while per normal practice, the cutoff frequency is defined as being down 3db at the cutoff freq.

I compare yours to a 2pole Butterworth (purple trace) and a 2pole Chebychev (lt. blue trace) as computed by FilterPro, a free download from TI's website.
 

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audioguru

Well-Known Member
Most Helpful Member
what do u mean by droopy? i figured this filter is crap, but i had to start somewhere.
If your filter is designed for 100Hz then its response at 100Hz is down -6dB. A Butterworth 2nd-order filter is down -3dB.

Your 100Hz filter attenuates frequencies as low as 20Hz but mine is flat to about 60Hz or 70Hz.
Mine has the correct slope at about 130Hz but yours still doesn't have the correct slope below 300Hz or 400Hz.
 

D.M.

New Member
thank you mike. i do understand these concepts but it is just a lot of things to look at and know off top.

im still learning obviously, so i do appreciate everones response.
 
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