From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2001-06-10 12:53:34
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
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