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public

join:2002-01-19
Santa Clara, CA
reply to lutful

Re: EEG and EOG help

said by lutful:

said by public:

This will not work

Have a look at the captured EOG waveforms from a crude breadboard implementation of that "textbook" circuit in this project report from which I copied that diagram.

I agree that the input can be improved to filter out noise above 100Hz.

Well working means working not just in a shielded room.
Bio signals are typically uV range, in common mode noise of several V. So max cmrr is essential.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

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said by public:

Well working means working not just in a shielded room.

Of course I agree with you ... but popular "eye-control" science projects using that basic design somehow work with buzzing lights, wifi hotspots and cell phones in high school gyms.

Anyway, in this thread and a previous ECG/EEG thread, I mentioned there are integrated circuit front-ends for ExG signal pickup from companies like Analog Devices. They take care of component matching and provide very good CMRR.

said by public:

Bio signals are typically uV range, in common mode noise of several V. So max cmrr is essential.

I commented on actual EOG amplitude/frequency earlier and mentioned that it is quite different than ECG. Here are two charts which provide characteristics of various bio signals and the main considerations for amplifier design.

You will see that EOG is actually quite robust requiring mainly good DC response, reasonably low drift and of course very high gain. CMRR and noise (even 50/60Hz) is less of a concern with EOG than other bio signals.

However, I agree that you could come up with a much better (discrete) front-end circuit for EOG than the two examples posted so far in this thread. My suggestion is to search for an IC that does the job.