Re: 24 channel EEG

Is there a need to edit a open source book? To collect all the years of worth. Can publish it in cc-zero I think (public domain)


Best Regards,
Andreas E. Sjöstedt

Date: Sat, 11 Jan 2014 23:26:20 +0200
From: [email protected]
To: [email protected]
Subject: Re: [Openeeg-list] 24 channel EEG

Justin

There are over 10 years worth of posts in this forum and these sort of things have been discussed over and over.  Yes, searching inside this forum is difficult, but if you do an intelligent search via google, you should easily get hits about any subject regarding openeeg on one of the mirrors.


All 2-channel EEG amplifiers are bipolar.
All COMMERCIAL many-channel EEG amplifiers (16, 21, 24, 32, etc.) are unipolar.

Building a many channel device based on the bipolar openeeg design is akin to bolting two motor bikes together to build a car.


A motor bike has 2 wheels.  A car has 4 wheels.  Both have been optimized for their respective role in the design phase.  Yes, you can bolt two motor bikes together to build a 4-wheeled vehicle, but I would not call such vehicle a car.  You would also be rather silly doing so.


Here are just some reasons why many-channel amplifiers are unipolar/monopolar:

1. Instrumentation amplifiers such as the AD620 or INA128 are expensive, if you need 24 of those you amp would be unnecessary expensive to build.  You would anyway loose the INA's high CMRR advantage, due to the way you would use the many-channel amplifier.  Many-channel amplifiers use good quality opamps.  Much more cost effective than INA's.


2. You would need a minimum of 49 DIN connectors, assuming that you would use medical-save/grade connectors.  Again, so many DINs would be rather expensive and they would need a large front panel to carry them all, DINs are big connectors.


3. The user would HATE a many-channel bipolar design.  He would need to jumper all these commons (the CHx(-) ) together.

4. The DRL circuit is just totally unpractical to sum from so many bipolar channels.  Any bipolar inputs that are used in the DRL summing circuit would either HAVE to be used, or jumpered to ground to prevent floating inputs to saturate the DRL circuit.  A many-channel amplifier needs it's own, specific DRL, different from what is used in a typical bipolar design. 


Please note this:  Some commercial many-channel amplifiers can be run in bipolar mode.  But the design is unipolar.  All they do is use 2 monopolar channels and subtract one from the other (normally in the digital domain).  Often, this can only be done on a handful of channels, say the first 4 or so.


Regards
Stefan



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