I designed a EEG amplifier circuit for for EEG signal amplification and filtering.
Because I never had any experience in filters, It would be highly appreciated if someone could check my design.
In general circuit looks like this: AD620->Driven Leg->DC-restoration(offset correction)->two passive filters(low and high pass) before amplification->regulated amplifier, gain 1..25->2nd order active high-pass, gain 1.5->4th order active low-pass, gain 30->passive high pass->to ADC
50 hgz hum cancellation done with Driven Leg. Offset correction schematic, DLR block and AD620 are not shown.
The first my concern is oscillation.
I am going to use TL084CN op-amps. And their oscillation can be a problem.
The second my concern is filter design. I used online tools for filters design:
According to okawa-densh tool some of the filters will oscillate. For e.g. Oscillation frequency for 1-st stage of low-pass second order filter:
f = 71 [Hz]
For High-pass filter:
Is it normal?
I simulated circuit in LTspice, images below. Graph is frequency response. Lines on graph marked with dots on the schematic.