From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2001-06-08 17:48:44
Hi yaniv,
----- Original Message -----
From: <yaniv_vi_at_yahoo.com>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Thursday, June 07, 2001 6:41 AM
Subject: [buildcheapeeg] Re: input stage safety question - for all the electronic engineers
...
> 1.what the shield is connected to - is it vcm ?
Basically Yes. It is the buffered common mode voltage
of each amplifier channel.
The shield driver output is 1/2*(Vp+Vn)
(with Vp = Voltage at positive electrode port, CH1+ or CH2+
and Vn= Voltage at negative electrode port, CH1- or CH2- )
> there's any way to check for shoring of main pin to shield -
> do some kind of a test circuit that online monitores this ?
Testing could be done with a simple ohm-meter.
An onboard testcircuit that monitors electrode impedance would probably
detect the shortcut.
However I have a more simple solution:
change R21 and R23 to 10 meg and bypass each with 100nF .
This will limit the DC current through the test person to harmless values
in case of a defective electrode cable.
> less preferable is some way to connect beofre using the electrodes
> to some internal testing device .
>
>
> BTW why this phenomemn happens ?
If the cable is frequently subjected to mechanical stress (sharp bending etc.)
the isolation between shield and inner wire could break down.
As a result the output of the shield driver OP amp (IC2A, 5A) is fed back
to the input. The open loop gain is 1 at 0 degree phase angle. This is a very
unstable condition leading very quickly to +5V or 0V output.
Regards,
Joerg
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