From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-01-10 20:30:34
Hi,
----- Original Message -----
From: Jim Meissner <jpmeissner_at_mindspring.com>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Tuesday, January 08, 2002 3:59 PM
Subject: Re: [buildcheapeeg] Re: Input protection
...
>The opto isolator is what must be excellent to protect the patient. That is
>where the effort must be spent.
> Joerg says it is too simple to even worry about.
Can you please quote the mail where I have said this ?
I can not really remember that I have said that opto-isolation is
trivial.
In fact the modularEEG opto-isolation in its current state will
only withstand about 1000VDC and perhaps 400VAC.
(The advantage (at least for me, beacuse I like it)is the battery
less power-supply. I think Dough likes it too, because he can power
the whole system from his 5V power supply...)
The still unanswered question is, whether there is a non-medical
norm that permits this isolation voltage or if we really
must fulfill the IEC601-1 with its about 6000VAC test voltage.
_If_ we must comply with the IEC601-1 isolation voltage, the
modularEEG isolation has to be changed as proposed by different authors
here in the group.
However this implies battery operation (that I personally do not
like)
I have put emphasis on protecting the patient from input stage failures
in my last postings.
Obviously you you do not see any danger for a patient getting between
V+ and V- .
IEC601-1 is a bit more restricitive.
It is quite a different thing if you touch a +-15V DC supply with dry fingers
or if you are connected to this supply via low impedance electrodes.
IIRR Vladimir has tested his EEG electrode impedance (pasted to his head)
with a normal Multimeter in Ohms-Mode (using a small DC current
source) and got a nasty shock, seeing light-effects.
> If it is so simple lets see it.
>
>I looked at your receiver design. My main concern is that the base of Q2 is
>left floating and leakage current and noise
There could be some susceptibility for capacitively coupled line-hum.
>at higher temperatures may
>inadvertently turn Q2 on.
Does this refer to the reverse current of D1 or to Q2 alone ?
>A 10 k or so from base to emitter would prevent
>that.
It would reduce the input impedance to below 10k and reduce
susceptibility for capacitve signals on the other hand it would
bypass some part of the photodiode current. Does the amplifier
have enough gain reserve to handle this ?
> Has Joerg looked at this design?
Yes. Just done that.
>I would be interested in his
>opinion.
To say more, I would have to
download the datasheets of the PD15-22 and BC847 ...
Regards,
Joerg
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