Re: EEG-SMT modding

This is a school project, so you will benefit most by investigating and
studying various theories and learning how this works rather than if I just
tell you what you need to do and you do it.

I recommend you assemble the kit as specified and get it working
correctly.  Then improve it as you see fit.  If you want to lower the
cutoff frequency of the filter, study active filters.  There are many web
sites about this.  Regarding substituting the instrumentation amplifier, I
recommend using the original part and getting everything to work correctly
first.  If you want to substitute better parts to improve the design, do so
after you get the original working.  To help you convert to a new
instrumentation amp. study instrumentation amplifier applications.  All of
the analog IC manufacturers have plenty of info on this.

Good luck!





Kind regards,

Greg Feneis

 <http://www.linkedin.com/in/electromechanical>




On Sun, Jul 23, 2017 at 11:25 AM, Damián Filo <[email protected]> wrote:

> Thanks for reply.
>
> So, the cutoff frequency, do you know how exactly should I set the
> resistor and capacitor values for 50Hz?
>
> EEG-SMT is using ATMega16, so 8 channels shouldn't be problem, but I'm
> worried about the firmware capabilities and I think, that it allows only up
> to 6 channels. So I need to check this thoroughly.
>
> About the INA129, it is better, when I checked the datasheets, it is
> cheaper and better, but the inner resistor values are different, so I'm not
> sure if I shouldn't change some resistors or capacitors around it. Do you
> have any idea how can I check it?
>
> On 23 Jul 2017 6:57 p.m., "Greg Feneis" <[email protected]> wrote:
>
>>
>> ​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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