Re: Active Electrodes

Hi Tonio

1. Your pin defintion on the 3.5mm plug is wrong.  The front (nose) pin is
+5V as you have noted.  But the middle contact is the signal and the 3rd
contact is the GND.

2. I would suggest that you add a 100 ohm resistor in series with the +5V
at every 3.5mm socket.  Olimex has done this on there SMT version.  The
reason for this resistor is just simple current limiting, should a short
circuit develop inside that jack plug.  These plugs are tiny and fiddely,
and especially if you solder them yourself, short circuits can happen quite
easily.  Especially after a lot of use, with cable flexing, etc.

3. You mention powering from an "external power source".  This power source
should be a medical grade power supply.  I don't think you use such.  If
you don't use batteries, make sure that your source has at least 3000VDC
isolation from the mains.  The simplest way would be to use a similar DC/DC
isolated converter as what is used on mod/SMT-EEG, i.e. TMV0505S or similar.

4. The neatest and best way to power the actives would be to use the same
isolated power as from your modEEG front-end, i.e connect to +5V/2 and AGND
as per modEEG schematic.   Each active has a TL062, so it will draw about
0.5mA.  So even the maximum of 12 active electrodes on your 6-channel amp,
should add no more than 6mA.  The TMV0505S should still be fine with this
extra load.

5. Yes, connect all grd/shield pins together and connect to AGND.  In this
way you also have a common setup that is compatible with both active and
passives electrodes.

Regards
Stefan

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