From: peterson_at_d...
Date: 2001-06-11 01:57:50
--- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann" <info_at_jhansmann.de> wrote:
> Hi Grant,
>
> ----- Original Message -----
> From: Grant Bright, Ph.D. <gbright_at_b...>
> To: <buildcheapeeg_at_yahoogroups.com>
> Sent: Sunday, June 10, 2001 6:16 AM
> Subject: Re: [buildcheapeeg] Re: newest analog schematic and parts
list
>
>
> ...
>
> >If someone wanted more input channels, say eight or perhaps even
up to 20, can the circuit itself be >scaled up to acommplish this?
>
> The analog circuit is not really the limitation. Each additional
pcb gives 2 additional
> differential input channels.
> However care should be taken if all these differential channels are
tied together
> at their chX- and connected to the reference electrodes (earclips)
to get
> a monopolar signal takeup. If this were the normal setup it would
be desirable
> to buffer the reference signal with a separate amplifier, because
each differential
> channel connected to reference would load the reference voltage.
With few channels
> this would be no problem. For high channel counts however this
issue should be
> considered.
>
>
> Another point is the numer of multiplexed ADC-channles of the
digital board.
> The AT90S4433 can only handle 6 analog channles (at 10 bit
resolution)
> with the internal multiplexer.
> An external 1:16 multiplexer could be added. However the sample
speed of the
> AT90S4433 is limited as is the bandwidth of the RS232 connection. So
> 16 channles with up to 16 bit (10 bit used) at 256 Hz sample rate
would be
> near to the limit of an uncompressed data stream over a 115200baud
RS232
> connection.
>
> That means: Each 16 channels would require a separate RS232
connection.
>
> HTH
>
>
> Regards,
>
> Joerg
Even a 6 or 8-channel machine could be very interesting. Readings at
F3, Fz, F4, T3, T4, Cz, O1 and O2 would give you a lot of
information, for example. Or, for suspected conditions such as ADHD
or depression on a 6-channel version, you might not bother with the
occipital sites.
A few years ago a company called Aquathought put out a 16 channel
QEEG machine for $2400 or so (but actually about $3k by the time you
bought the needed QEEG database and peripheral equipment such as the
Electrocap. I remember seeing it, I think at SNR in '97, where
everyone was drooling over it. The machine has disappeared from the
market, though, and I never really heard why--whether they didn't
sell for some reason, whether the parent company went broke, or
whatever.
For standard montages, 19 or 20 channels would be nice. What about
the parallel port? I know, 8 opto-isolators, but for this it would
be worth it. This one probably really would need the medical
certification, though. But, come to think of it, a lot of us are
trained as researchers and could most likely put together the
necessary studies and field trials, which should reduce the cost
quite a bit. I could probably recruit a post-doc or 2 to do it.
Maybe Bob Thatcher would license us for limited use of his QEEG
database if he could see a large demand for the database once a
machine like this hit the market.
But this is for the future. For now, the 2 channel machine would be
wonderful.
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