From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-02-10 15:05:16
Hi,
----- Original Message -----
From: sleeper75se <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Saturday, February 09, 2002 11:17 AM
Subject: [buildcheapeeg] Re: EEG Electrode DC voltage
...
> great job! Bad results. :-) But now we know more.
A possible source where the DC offset comes from is the
input stage bias current that charges the electrode surface
by electrochemically altering it. If this mechanism can be
confirmed there are several approaches to avoid this charging:
1) AC coupling (not recommended due to noise problems)
2) disposable electrodes
3) aggressive (abrasive) cleaning of electrodes after use.(will shorten lifetime of electrode because it gets thinner with each
cleaning )
4) manually changing electrodes from + to - amplifier input, so that over time of use
the charge neutralizes.
5) automatically exchange the electrode assignment to the amplifier
input ports using a multiplexer at the input and output. (like a chopper amplifier)
> Please try
> chloridizing the electrodes too ...
At first I thought: No problem, but when I read an old "elektor"
article (german edition, no. 196, p 68, april 1987) about chloridizing
silver electrodes some questions arose:
What purity should the NaCl have ? Will normal houshold quality be enough ?
Do I really have to use destilled water for the saline ?
Is the recommended 0.9% concentration referred to volume or to weight ?
Do I really have to sacrifice a silver electrode for the Kathode
(The electrodes to be chloridized are attached to the Anode (+)),
or would graphite also be OK ? (I do not see any reason why the Na+
ions should be replaced with Ag+ ions ...)
BTW. I have found a bag with oudated ('97) disposable Ag/AgCl pregelled ECG electrodes (Type ARBO0H66 from ARBO) and will look what
DC offset these
have.
Regards,
Joerg
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