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Few questions on this active electrode circuit

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C5 - blocking capacitor in + power line
R4 - Defines the output impedance of the amplifier on 100Ohm
C1,C2,R3 - attempt about LPF ,cutting off HF
but I'm not sure if it makes sense, especially in conjunction with TL062
 

C1,C2 appear to be LPF at 200KHz which may improve AM radio rejection but the positive feedback resistor, I am not sure about.

Since EEG is ULF band why not remove this and change the gain to 500 instead to reduce the bandwidth to act as LPF.

Also since common mode noise will be high , include an ac cm choke on all cables. Small 100 pF additional output load could be added as well if you are getting RF noise.
 

Thank you.
Could you tell me what kind of filter is that?
 

Op Amps have internal cap to stabilize unity feedback designs but this gives fixed gain-bandiwidth product. Since your BW is < 50Hz your opportunity for LPF exists if you choose high gain. Since EEG is low signal and you ar using high CMMR IA chip, you can take advantage of this opportunity. but still may need ferrite sleeves for AC hum etc.
 

How do you calculate the cutoff frequency of this filter? Why is there capacitors and resistor on positive feedback?
 

The positive feedback may be an attempt to improve the phase margin stability so the unity gain does not oscillate at MHz rate. T=100pf * 10k =1us. Perhaps it offers minor attenuation of strong Am radio stray signals.

It has no effect on EEG signals.
 

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