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. circuit Frequency response

Thank you.

  • \$\begingroup\$ TL084s are terrible op amps for a 3V split supply. \$\endgroup\$
    – Matt Young
    Apr 8, 2015 at 12:26
  • \$\begingroup\$ For supply I am going to use 6x1.2 batteries. And split it with TL084. Like this: db4c15cb-a-62cb3a1a-s-sites.googlegroups.com/site/chipstein/… \$\endgroup\$
    – Andrew
    Apr 8, 2015 at 12:45
  • \$\begingroup\$ Is OP07 good choice? What can you recommend instead TL084? Thak you. \$\endgroup\$
    – Andrew
    Apr 8, 2015 at 12:56
  • \$\begingroup\$ Both suck equally for low voltage power rails. I would start with a LMP7704. It's not as fast, but has rail-to-rail I/O. \$\endgroup\$
    – Matt Young
    Apr 8, 2015 at 13:06
  • \$\begingroup\$ You shouldn't necessarily consider DRL to be the same as hum cancellation. \$\endgroup\$ Apr 8, 2015 at 13:45

1 Answer 1


"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: -

enter image description here

Clearly this circuit doesn't oscillate but, all the same, it has an oscillation frequency.


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