From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2001-12-15 00:50:52
Hi Andreas,
----- Original Message -----
From: sleeper75se <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Friday, December 14, 2001 10:08 PM
Subject: [buildcheapeeg] Re: Necessity of ir isolation
...
> > The difference-mode impedance is really reduced to 10Mohm - however
> this
> > type of impedance will only result in a minor amplification error
> > (about 0.1% loss) due to a virtual voltage divider between the
> > electrode impdance of ca. 10k and the diff-mode input impdance of
> 10M.
>
> But what happens if the electrode impedances are higher, and are
> unequal?
If the electrode impedances get higher e.g. both 50k , the loss
will increase to 1%.
CMRR will not be decreased because it simply has nothing to do
with difference mode input impedance.
In the MICROSIM simultaion I have used imbalances of 5k : 50 k and got
good CMRR.
> In message 1340, Jim Meissner wrote:
> > The other problem is the electrode source impedance. When you start
> > it is easy to get 10 K ohm with conductive paste. For medical or
> > clinical settings that is no problem. But if the person tested goes
> > into an altered state, the skin resistance changes and the
> conductive
> > paste has no effect.
> [snip]
> > In a meditative state the skin resistance will rise to 100 K ohms
> and
> > sometimes several meg ohms.
I know the effect that skin resistance rises to rather high values
from experiments with GSR-Meters, however the electrode setup is
completely different from EEG.
So I am not sure about the general validity of this information.
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)?
> Wouldn't higher input impedance (difference-mode) help reduce the
> problem of large electrode impedances as the voltage-divider effect
> would be much smaller?
Yes. If the electrode impedances were really as high as mentioned
above also a high differential mode input impedance would be very
important. Also I would recommend to replace the INA114 with a FET-
Input type.
However I am not convinced until now.
> Biosemi's approach to high-pass filtering on the input stage retains
> the higher difference-mode impedance. I'm sure you have read it
> before:
>
> http://www.biosemi.com/publications/artikel1.htm
Unless you want to build active electrodes the schematic shown
in the article has no advantage in a non-active
electrode setup (as intended for the modularEEG):
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.
BTW: The INA114 has laser trimmed internal resistors and a
circuit that inherently compensates tolerances - resulting in
an excellent CMRR of 115dB.
3)Further you can not use this design with driven shielded cables,
because there is no output with gain=1 in this circuit.
> Since they use a right-leg-drive, and not passive ground,
I did not find any hint in the above mentioned article that
this technique is used. Did I miss something ?
> 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 ...
> Biosemi solves this by turning the input stage off if the voltage at
> the ADC becomes too high or low.
>
> They give a brief description here,
>
> http://www.biosemi.com/faq/limit_current.htm
>
> but do not explain how they tell the difference between large input
> signals that cause clipping and a fault condition... :-p
Interesting.
> What do you think, could we use the Biosemi design? Not like their
> active electrodes, but as a regular input stage.
I do not think that http://www.biosemi.com/publications/artikel1.htm
shows the right approach for non-active electrodes because of the
above discussed problems.
Regards,
Joerg
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