From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-02-08 23:55:52
Hi,
in order to check if it is possible to increase the gain of the first amplifier
stage to G=100, I have just checked the DC output of the INA114 in my
comEEG prototype:
At G=10 the INA114 output showed up to 0.8 Volts DC when electrodes
(pure silver, shielded) were attached to the skin with 10/20 paste.
When the electrodes were shorted, the voltage dropped to 0 V. After removing
the shortcut, the voltage slowly increased over a few minutes until it
reached 0.5 Volts.
Referred to input this means an up to 80 mV DC voltage between the electrodes.
Probably an Ag-AgCl electrode system with saline electrolyte will show
less DC.
Due to this results I continue to claim that a versatile and robust
EEG amplifier should be able to handle more than +-100mV DC between the
electrodes without going into saturation.
Because of the 5V supply voltage and the limited output swing the
modularEEG input stage will probably stay at G=10.
Another solution (recommended by Jim Meissner IIRR) would be to increase
the supply voltage to +-15 Volts. At G=100 then +-100mV input DC would
produce +-10V output.
Regards,
Joerg
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