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TWIN T NOTCH FILTER TO REMOVE 60 HZ , harmonics questions

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integralx2

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I have a design , that has the right values for the cuttoff.


R1 = 39kΩ
R2 = 39kΩ
R3 = 9.1kΩ
C1 = 0.1uF
C2 = 0.1uF
C3 = 0.1uF


Center rejection frequency
f0 = 57.712584369[Hz]
f0 = 59.7382002206[Hz]

What about the harmonics that might exist ? What should I do to get rid of them or is there another thing I should add to filter harmonics? I will be making an ECG system, so i need to filter this cable noise, but I will be using shielded wiring.

Thanks
 
From a glance at your numbers, that filter will not work so well at 60Hz. Those cap values are too small, about 27 kohms at 60 hz.
 
ECG circuits use shielded cables and instrumentation amplifier ICs that have excellent common-mode rejection. The patient's right leg is usually driven with an inverted common-mode signal for cancellation.
 
Hello there,


Here is some analysis i did a little over 10 years ago. Note the relationship between the grounded component values and the others as the capacitor must be two times the value of the series caps and the resistor must be one half of the series resistors. Any deviation from that convention causes at least some detuning.

R4 and R5 were included in the analysis because in a real life circuit there is usually at least some output resistance in the circuit that feeds the twin T and some load impedance in the driven circuit.

I believe you can scale the components as follows:
The values shown are for a 60Hz notch, and if you divide all the caps by N, multiply all the resistors by N and that will keep the notch at 60Hz although that will change the input and output impedances of course. So for example if you divide all the cap values by 10 then you also have to multiply all the resistor values by 10 too.

In the following diagram the output is:
Vout=Vin*VoNum/VoDen

and you can set 'Real' equal to 1 to form the equation. That was included for people using hand held calculators way back when :)

Also note that in the graphs the peak phase characteristic is a little higher and lower than the graph seems to indicate. This was due to limited resolution in the graphing routine.
 

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hello,did you got the solution of remmoving 60Hz noise from the ecg signal so let me know about it i am also working on ecg signal
 
Hello,

The values shown in that drawing were for a 60Hz notch filter so that might help.
 
Mains hum is not only 60Hz. It has harmonics at 120Hz, 180Hz and many other frequencies.
An ECG circuit uses the excellent common-mode rejection of an instrumentation amplifier IC plus feeding inverted common-mode AC and DC signals back to the patient (at the right leg) for mains hum cancellation.
 
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