Bandpass & notch filter probelm

Thread Starter

teen devil

Joined Apr 6, 2009
47
hi guys ,

i am trying to make a 2nd order sallen key band pass filter from 0.05hz to 500hz .. when i am trying to simulate it my high pass is working fine and blocking the frequency at 0.05hz but my high pass filter isn't blocking at 500hz . sample goes for my notch filter i am created a 50hz notch filter in bode plot there is perfect 50 block but when i am giving the 50hz its not blocking . i am attaching both screeshots ..

will waiting for your help guys .. or if anyone have a 2nd rder filter schematic please share
 

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ronv

Joined Nov 12, 2008
3,770
You need both plus and minus 12 volt supplies. Or you need to bias the input to Vcc/2 and ac couple it.
 

Thread Starter

teen devil

Joined Apr 6, 2009
47
i've changed my approach i came across Texas instrument document about filters there i read about a different configuration about sallen key band pass filter ( screen shot attached) i ran the simulation on it using all the necessary voltage supplies getting a good amplification ..

now its stopping frequecy lower than 0.05Hz but still my Low pass filter isn't working properly it should be at 500Hz but not working .. am i doing the right thing .?

Thanks for your help guys i am new in this field.
 

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LvW

Joined Jun 13, 2013
1,279
Low pass filter isn't working properly it should be at 500Hz but not working .. am i doing the right thing .?
Why do you think the low pass "isn`t working properly"?
What do you expect from a second-order bandpass?
By the way - show a more detailed BODE plot between 1mHz and 50 kHz (instead of Ghz).
 

Thread Starter

teen devil

Joined Apr 6, 2009
47
Try 29.4k for R1 and 15nF for C2.

Reduce the upper frequency range of your Bode plot to 1kHz for better resolution.
here u go changed the values what u values suggested according to formula of 1/2*pi*R*C low pass frequency should be 360 HZ but still its not blocking
 

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Thread Starter

teen devil

Joined Apr 6, 2009
47
Why do you think the low pass "isn`t working properly"?
What do you expect from a second-order bandpass?
By the way - show a more detailed BODE plot between 1mHz and 50 kHz (instead of Ghz).
cause i think it should attenuates or rejects any frequency more than 500hz which i set it with the resistor and capacitor values but it is allowing freq more than 500Hz to passu can see in the screenshot abv
 

LvW

Joined Jun 13, 2013
1,279
....... but still its not blocking
cause i think it should attenuates or rejects any frequency more than 500hz which i set it with the resistor and capacitor values but it is allowing freq more than 500Hz to passu can see in the screenshot abv
Please, be aware that there is no filter circuit that is able to "block" a frequency range. Your circuit shows the classical and normal behaviour of a second-order bandpass.
Please note that all frequencies beyond the upper corner frequency (500 Hz)
are attenuated by the filter circuit with a damping factor of 20 dB/dec only.
In detail: A frequency of 5 kHz is attenuated by a factor of 10 (20 dB) only.
 

Thread Starter

teen devil

Joined Apr 6, 2009
47
Please, be aware that there is no filter circuit that is able to "block" a frequency range. Your circuit shows the classical and normal behaviour of a second-order bandpass.
Please note that all frequencies beyond the upper corner frequency (500 Hz)
are attenuated by the filter circuit with a damping factor of 20 dB/dec only.
In detail: A frequency of 5 kHz is attenuated by a factor of 10 (20 dB) only.
oh i didn't knew tht .. thnx alot . so my 2nd order bandpass filter is working fine :)
 

Thread Starter

teen devil

Joined Apr 6, 2009
47
one last thing after designing my filters i tried connecting AD620 instrumentation amplifier to make ECG amplifier .. i have attached screenshot too . can you tell me why i am not getting good amplification . i think i am doing something wrong with AD620 .. i read the datasheet and attached RG = 100 ohms for gain of 500 but its still not working .

i would really appreciate the help
 

Thread Starter

teen devil

Joined Apr 6, 2009
47
sorry i am really at loss here .. i am new electronics fields can u please explain this in detail .. i tried connecting ground to -ve input of op-amp but it didn't work ..... i'll be really thankful to you if u explain the DC return path you are saying i am missing
 

Thread Starter

teen devil

Joined Apr 6, 2009
47
Have you looked at the datasheet for the AD620? Specifically page 14 of 20 in revision H.
I tried what u said i configured Inst. Amplifier with the right leg drive circuit as per datasheet . finally i started to get a good amplification tht's in screenshot 1 .. but when i am connecting my bandpass filter with the notch filter the voltage is dropping again to nV .. u can see in screenshot 2
 

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