Dave Van Horn wrote:
>At 10:59 AM 6/14/01 -0600, Victor Valdez wrote:
>>Is there anybody that can help me to design a pic based interface for a
>>brainwave monitor? I need at least a four channel display. It can be for
>>pc or gameboy.
........
>You have a major amount of isolation work to do for anything like this that
>connects to a line powered device.  Serious safety issues.
>
>That being said, you need differential amps for the signals, with wide CMRR.
>A browse through national semi' s web page should let you find some good
>op-amps to do the job.
>The most bandwidth you need is only a few hundred Hz, so you can really
>compromise on speed.
>

Victor,

EEG signals are very small, in the 5-100 uV range. You will need a
differential amp, a low-pass filter, very good electrodes, and a
well-filtered power supply, in addition to isolation as Dave said.

Anything you connect to the body has the definite possibility of
shock hazard, so this should be your first concern. Running your
amps off batteries and feeding to a gameboy or battery operated
notebook is a good idea.

The amp should have the highest CMRR possible and preferably be
battery run. 60 [or 50] hz pickup from the mains is a big problem
with this type of recording, due to the low signal levels. You
should try to put a lot of gain in the headstage, as opposed
to downstream, because this usually results in less overall
noise. You can use a "driven shield" on the electrode wires.

You also need a very good ground/reference lead on the scalp,
located near the recording electrodes. It's exact placement
has a lot to do with the type of recordings you end up with,
and you might do some research on what is called the "10-20"
EEG arrangement.

The electrodes should be high quality silver/etc, and
contact made using conductive gel. They should be taped down
firmed to prevent movement artifacts. The technicians usually
clean the oils off of, and rough up the scalp surface a little,
to get good contact. If you head is covered with a bunch of
tiny little welts afterwards, the technicians know they did
their job correctly.

The headstage needs some kind of AC-coupling in order to
reject the contact potential of the electrodes. You have uV
level signals riding on a time-varying contact potential that
is on the order of 0.5-0.7v.

Plenty to think about, good luck,
- dan michaels
www.oricomtech.com
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