Re: [Openeeg-list] active electrodes - further research

[Jarek <jf...@ya...>]

> > Today I connected my active electrodes to forehead (still the same prototype as
> before). 
> 
> Can you say something about the connection from the metal plates to the skin? I 
> guess you use no preparation or 10/20 paste and just some pressure that holds 
> the electrodes in place?

Yes that is correct. I use a headband where the electrodes are attached. There is a good
presure there. I use it for both for passive and active electrodes. I do all my
measurements on forehead now, so that is not a problem.
I do not have any paste, for passive electrodes I use a water with salt, but for active
usually they are dry. 

> 
> > I think it doesn't look very good. I am not even sure if that is eeg (but what it
> could
> > be).
> 
> > 
> > The signal does respond to DRL electrode, because I am able to see 60Hz interference
> > disappearing when I touch DRL electrode.
> 
> 
> BTW: A good user connection with VGND or better DRL is no option but a _must_ 
> for this type of design, because it needs a bias current return path. If this 
> path is not provided you most likely will get nonsense readings.
> 

Yes i know, and I do use is. I see the difference before and after DRL (or VGND) is
touched, but all my comments (and pictures) describe time when I am connected to DRL.
I tried also connect to VGND. It is similar like with DRL, but 60Hz are not supressed.

I also tried one finger to VGND and one do DRL in the same time, and the result is like I
was connected to DRL.

> 
> > But, as you can see, the signal is very "jumpy".
> > To me it looks like the DRL can't stabilize it enough
> 
> The DRL does not stabilize the signal.
> In fact the DRL loop does nothing to the differential EEG signal.
> 

Hm. It is now very difficult to keep the signal within the range (y axis). It often jumps
behind this range. 
From my obserwation looks like DRL helps somehow keep it within the range, but it is
still not enough.

Also after I release and then touch DRL again, it takes a few seconds with flat signal
(on max or min signal) until it appears again.

> It only removes most of the common mode voltage (e.g. 60Hz hum) that is not part 
> of the signal.
> 
> > (although it does stabilize 60Hz).
> > 
> > I made several measurements with both dry and wet electrodes (active), and the signal
> is
> > always like that, so that is not because of skin impedance.
> 
> Strange.
> 
> What signals do you get when you connect your actitve electrode inputs 1)directly
> 2)with 1meg
> 3)10meg in series with each input to DRL?
> 

I will do that today in the evening when I am at home and will let you know.

> > I also measured eeg with passive electrodes and it looks ok. I was able to notice
> teeth
> > grinding on it 
> 
> OK.
> 
> >(it was not possible with active electrodes).
> 
> Not OK.
> Something is quite wrong if such strong signals can not be measured.
> 
> Can you still get the calibration signal? Or could it be that the TLC272 inputs 
> have been damaged by ESD due to no input protection?

I do not believe that because I do see significant difference when I touch DRL (60Hz
suppression visible on FFT chart). I guess it they were broken, that would do nothing.
But I will check them again today evening.

I also would like to create another test circuit (generator), like calibration signal but
independent from modularEEG and with variable (programable) frequency to see if that
makes a difference. I want to do this with parallell port of PC, but first need to learn
about how to do this easily (I mean how to access parallel port from program and what is
the pins layout), so it may take some time.

> 
> Without any input protection I recommend to attach the DRL or VGND electrode 
> first and then the actice electrodes, so that electrostatic charges are removed 
> before they can do any harm.

I agree, but that would be technically more difficult. Can you tell what level of
electrostatic charges can we expect on head?


I am thinking loudly here. The electrodes (amplifiers) are not identical. Also there are
probably different capacitances (Cs on you schematic), I guess they can couse slightly
different responses? I know that better protype could help here, but I guess two active
electrodes will never be identical. Not sure how that make a difference.

Thank you.
Jarek