Re: EEG-SMT modding

​See reply inline below​





Kind regards,

Greg Feneis





On Sun, Jul 23, 2017 at 4:32 AM, Damián Filo <[email protected]> wrote:

> Hello, I am building my own modded EEG SMT, and I noticed, that there and
> also in original OpenEEG is "3rd order Besselworth filter" with  Fc=59Hz. I
> am not sure what Fc stands for, but if it is something like cut-off
> frequency, it won't filter 50Hz electrical hum in europe. Shouldn't it be
> then set to 49 Hz, or something like that?
>
​Yes, either cutoff or corner frequency or some such.  It's a low-pass
filter that attempts to block frequencies higher than 59Hz.  You can tune
the filter to suite your particular needs.  Keep in mind that some EEG
signals are near 50Hz.  As you lower the cutoff frequency to reject power
line noise, you also begin to reject EEG signals.  This is not a problem if
you are only interested in lower frequency EEG signals.  ​


>
> I am making my own version of EEG-SMT mainly because of being able to make
> the PCB at school conditions (It's a school project) and also usage of
> lower cost newer parts like INA129 instead of INA114, different 3,5"
> jacks... Also I'm making it scalable, so it would be possible to connect
> extension module through 14pin idc connector (up to 8 channels would be
> then possible). It would be also possible to connect bluetooth module
> instead of USB/UART and the power would be supplied through microUSB
> connector.
>
​Regarding 8 analog channels into a single microcontroller's ADC, there is
likely a limit to the number of analog channels you can successfully record
simultaneously with the original processor.  I don't know what that is.  As
you may know, each conversion of a channel's analog sample takes some
time.  Add to that, changing analog channels input to the ADC, add to that
time required for the microprocessor to fetch the conversion from the ADC
and save it or send it somewhere.​


>
> I'd be glad to send schematics and PCB design in Eagle to anyone who would
> ask for it. Now (23.7.2017) I'm at 60%.
>
> Also I have one more question, will there be a problem when I use INA129
> instead of INA114? I've noticed, that INA129 creates less noise and is
> almost the same as 114 and it is cheaper.
>
​Please review the datasheets of both parts and use the part with better
specifications, even if it is more expensive.  Unless the cost difference
is extreme, I would only be concerned with lowering cost if I were going to
go into production for sale.  In that case, I would get the system working
exactly as shown in the schematic first, then I would experiment with
tuning and changing parts to suite my needs.  ​


>
> Thanks for replies.
> --
> Damián Filo
>
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