From: peterson_at_d...
Date: 2001-05-23 03:49:51
--- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann" <info_at_jhansmann.de> wrote:
> Hi,
>
> ----- Original Message -----
> From: Waldemar Neto <wpneto_at_o...>
> To: <buildcheapeeg_at_yahoogroups.com>
> Sent: Tuesday, May 22, 2001 5:24 AM
> Subject: Re: [buildcheapeeg] Re: electrodes
>
>
> >Most of professional equipment usually has an electrode tester ,
which we can use for
> > test or include in our cheapEeg "Aparatus" :
> > the measures of resistance of each electrode shall be less than
3 kOhms
> > (from ground in monopolar derivation ) - preferably less than 2
kOhms .
> > In a few cases where the signal is very weak it mays be necessary
less than
> > 1 kOhm - the ideal - ( like "brain silence" by tumors or CET,
extremely fat
> > bald skin, anencephalia and other )
>
> The input of the RS232EEG03analog is designed to handle electrode
impedances
> up to 50 kOhms and heavy asymmetries in impedance. This is possible
through
> shielded electrode cables with driven shields and a very high
common mode
> input impedance and the common mode voltage compensation by the
> right leg driver.
>
Yes, excellent. Lexicor has a built-in electrode impedance tester,
and their written literature recommends no more that 10kOhm
impoedance, and stick to one kind of electrode, etc. but verbally in
their training workshops they tell you 20k is OK and for ordinary
training purposes you can even mix electrode types. With the kind of
tolerances you describe here you could use almost anything for
electrodes and get reasonable results. I think we will be free to
experiment with electrodes and people will end up using whatever kind
they find handiest, cheapest most comfortable, or whatever.
Personally I like the little Grass 10 mm cups because they have such
fine (thin) and flexible leads. I use 10-20 paste for scalp
adhesion, but sometimes use Nuprep for the ear clips--that's right--I
mix electrode paste types, have done so for years, and can't tell the
difference from my instrument readings.
> >Do you think that would be interesting to include a little level
( Ohmmeter ) with LED
> > activated threshold for that electrode levels ?
>
> A simple Ohmmeter will not do the job very well. Normal Multimeters
> use DC current to measure resistance. DC would however polarize
> the electrodes and possibly violate IEC601-1 that demands a maximum
> of 10uA DC flow through the user.
>
> I have thought about a circuit extension for measuring the
electrode
> impedances (complex AC resistance). A usual way seems to be by
> injecting a 30Hz 1uA current into the amplifier input and the
electrodes
> and measure the voltage drop over the electrodes.
> During the impedance measurement no brainwaves could be sampled.
>
> However there seem to be devices that can simultanously monitor eeg
and
> impedance. I assume they use a current injection with a frequency
above
> the anti-alias filter cutoff frequency and some advanced AC current
sources
> with a output-impedance greater 10^10 Ohms, so that the impedance of
> the eeg-amplifier input will not be degraded.
>
> Regards
>
> Joerg
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