Topic 'Re: EEG and EOG help' in forum 'Electronics' - dslreports.com

Re: EEG and EOG help

Sun, 25 Nov 2012 22:01:02 EDT

lutful posted : Analog Devices AD8295 (for example) incorporates a precision instrumentation amplifier + dual op amps + 2 matched resistors. »www.analog.com/static/imported-f···8295.pdf

*** National Instruments EyeMario EOG solution:
»www.ni.com/white-paper/11981/en#toc2

AD8221 in-amp > AD8675 for gain+filtering > AD7401 (galvanically isolated ADC)

Re: EEG and EOG help

Sun, 25 Nov 2012 20:55:45 EDT

cowboyro posted : Besides the instrumentation amplifier you need galvanic insulation from mains.
Even a 60dB CMNR brings the CM noise rejection to levels comparable to the useful signal. Look at the dedicated insulated amplifiers, me bets one of those will work ;)

Re: EEG and EOG help

Sat, 24 Nov 2012 16:56:18 EDT

SparkChaser posted : Thanks again for the comments. Many of the circuits I've seen have an instrumentation amp as the first stage. Usually the INA118 series

»www.ti.com/lit/ds/symlink/ina118.pdf

Re: EEG and EOG help

Sat, 24 Nov 2012 10:28:05 EDT

lutful posted :

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. :)

Re: EEG and EOG help

Sat, 24 Nov 2012 03:54:38 EDT

public posted :

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.

Re: EEG and EOG help

Thu, 22 Nov 2012 09:57:56 EDT

lutful posted :

said by public:

This will not work

eog_project.pdf

Re: EEG and EOG help

Thu, 22 Nov 2012 06:45:34 EDT

public posted :

said by lutful:

said by SparkChaser:

There are a few circuits floating around the net. Pretty much all the same functional blocks

I think some of those circuits came from ECGs. EOG signals are smaller in amplitude and lower in frequency.

This will not work. You will never match the resistors in the input for acceptable cmrr.
Also need low pass on the input to prevent cm voltage driving the amp into the rails.

Re: EEG and EOG help

Wed, 21 Nov 2012 12:52:39 EDT

lutful posted :

said by SparkChaser:

There are a few circuits floating around the net. Pretty much all the same functional blocks

I think some of those circuits came from ECGs. EOG signals are smaller in amplitude and lower in frequency.

You may recall there was an ECG discussion last year where I mentioned there are dedicated ICs which incorporate most of the front-end signal processing. Please have a look at Analog Devices.

said by SparkChaser:

I'm starting to think I need better electrodes and placement.

Most probably skin oil has accumulated on them. You could clean up with alcohol and apply some gel.

EOG-based computer goggles actually place the electrodes almost ideally. The reference electrode smack in the middle of the forehead and the left/right electrodes equal distance apart from that on the temples.

Re: EEG and EOG help

Wed, 21 Nov 2012 07:33:53 EDT

SparkChaser posted : Thanks lutful, yes that is the one that got me started. It's not for me but for someone that is paralyzed.

I can't get a decent signal out of his circuit. He has some strange values in his notch filter which doesn't make it work very well. I modded the filter part. Still not getting good signals. There are a few circuits floating around the net. Pretty much all the same functional blocks. I'm starting to think I need better electrodes and placement.
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"Facts do not cease to exist because they are ignored." - Aldous Huxley

Re: EEG and EOG help

Tue, 20 Nov 2012 23:13:46 EDT

lutful posted : Since nobody has responded so far ... please have a look at what this guy has done to get decent EOG output with cheap silver electrodes.

»makeprojects.com/Project/Eyeboar···/1#s8781

P.S. I recall you occasionally visit Ottawa. My wife is an ophthalmic medical technologist just in case you need a professional EOG and any other eye checkups done.

EEG and EOG help

Sun, 18 Nov 2012 16:50:30 EDT

SparkChaser posted : Anyone done any work on EEG or EOG (electro-oculogram)? Specifically looking at EOG. So far, I get nothing but noise. Using a instrumentation amp and some filtering. Part of my problem is the lack of appropriate electrodes.

If you have succeeded, I'd like to know what you did.