From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2001-06-07 14:22:22
Hi,
----- Original Message -----
From: Psytrix Yahoo <psytrix_v_at_yahoo.com>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Thursday, June 07, 2001 3:57 AM
Subject: [buildcheapeeg] Re: EEG x ECG Right Leg connection
>Hi, Thanks for answering -
>The question which remains is : would be possible that the right leg connection
>could be more necessary only in ECG but not so in EEG
No. The DRL (driven right leg) is more required in EEG, because
the typical signals are much smaller than in ECG. Therefore
the Common Mode Rejection Ratio is more crucial.
DRL is definitely used in advanced EEGs:
(see Fig 3. : An example of a multichannel EEG amplifier with minimal noise,
a low parts count and a low power consumption. An extra intergrator has
been added in the feedback loop to redude the output offset. A DRL and
a guard circuit are used for maximal interference reduction.
http://www.biosemi.com/publications/artikel7.htm)
>(considering the legs
>bifurcation from heart horizontal derivations for ECG and
>the "head convergence" at the neck level )
The influence of the body geometry and different electrode positions
(for DRL-electrode) could be modelled and simulated with e.g. PSPICE
if data were available about the impedance of all body parts and skin.
However I do not have those data.
Until now I am using a simplified model that assumes, that all body fluids
form a low impedance (about 100 Ohms) electrical node that is connected
to the electrodes via a medium impedance (about 1..20k Ohms , skin to electrodes).
In this model bifurcation of the legs or convergence at the neck is
simply ignored.
> , or that right leg connection would
>be replaced for other locations like brow's or
>occipital electrodes ( for the same reason ? ) .
In my understanding (that is of course limited) the right_leg electrode
could be placed anywhere.
It is a sort of hi-tech replacement for the amplifier common or ground
electrode (in RS232EEG03analog this would be VREF/2).
BTW. where do e.g the brainmaster-users place the Ground-electrode ?
Regards,
Joerg
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