From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2001-05-22 11:26:06
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.
>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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