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:
http://www.analog.com/designtools/en/filterwizard
http://sim.okawa-denshi.jp/en/OPttool.php
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:
f=0.16 [Hz]
Is it normal?
I simulated circuit in LTspice, images below. Graph is frequency response. Lines on graph marked with dots on the schematic.
Thank you.
Best Answer
"Oscillation frequency" is a term that describes the frequency where the input-output phase relationship is exactly zero degrees i.e. if this filter were part of a circuit used as an oscillator it would oscillate at this frequency. Sometimes oscillation frequency is quoted at 180 degrees phase shift (inverting filter configurations)
It doesn't mean the circuit on its own will oscillate. Look at the results of an RLC low pass filter: -
Clearly this circuit doesn't oscillate but, all the same, it has an oscillation frequency.