From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2001-12-15 14:28:20
--- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann" <info_at_jhansmann.de> wrote:
Hi Joerg,
> 1)In order to get two bipolar channels (as in the modularEEG)
> the circuit above would have to be built 4 times (more than
> 4 x 10$ in cost) and in a further stage the difference
> between each two channles would have to be calculated.
>
> 2)Inevitable tolerances in the feedback networks (of the active
> electrodes)of lets say 1% would produce a very poor CMRR of
> only 40dB.
>
[snip]
> 3)Further you can not use this design with driven shielded cables,
> because there is no output with gain=1 in this circuit.
They don't tell the whole story in the article I sent you, so maybe I
was misleading, I'm sorry.
artikel1.htm depicted what could be used as one half of an in-amp.
In another article, they claim 136 dB CMRR for the complete circuit.
The feedback network differs slightly, but it's the same idea. They
don't use a symmetric setup as suggested by (?) artikel1, but a
reference electrode without any special feedback. Together with the
integrator-setup described in artikel1, it forms a 2-opamp
instrumentation amp. The signal from the reference electrode is also
fed to the shield driver and right-leg driver.
http://www.biosemi.com/publications/artikel7.htm
Here are direct links to the circuit(s) in question:
http://www.biosemi.com/publications/eegfig2.gif
http://www.biosemi.com/publications/eegfig3.gif
As you can see, you only need three pairs of opamps for two channels
(+ one additional shield
driver amplifier).
>
> > the only
> > problem with it lies in the unlikely event of one electrode lead
> > being shorted to V+ and another to V- or ground.
>
> Unlikely but more or less dangerous ...
Agreed. And I can't think of a good way to fix it either, except
those noisy resistors. By the way, 470k is a bit unecessary. Using a
power supply of +/-3V you only need 2*62k to meet the 50uA limit
requirement => 2 uVp-p current noise (actually a bit more since the
signals are not limited to 10Hz) for INA114. Is that low enough for
EEG? There are opamps with very low input noise currents available,
but they have higher voltage noise and require careful layout.
Example: http://www.national.com/ds/LM/LMC6001.pdf
First question last:
> Are any URLs / publications available that cover the above
> mentioned effect - e.g. show that properly prepared and
> attached EEG electrodes can get several meg ohms under
> certain conditions (deep meditation)?
Hmm, I don't know of any. Since this is the major concern for me, and
no more information is available, I suppose we need to Build and Find
Out.
There are a couple of other things I'd like to discuss as well, like
battery power, the virtual ground point and ADC noise. I hope you
have the time and inclination to do so. But that's for another post.
Regards,
Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:33 BST
