As we know each experiment has different impedance of electrodes. Different impedance implies different amplitude of voltage. So, my question is, is dependency of impedance on voltage significant? If yes, then how can we compare two signals of different experiment?
A well known clinical Ag-Ag/Cl saline system is that from EGI http://www.egi.com/ , their Geodesic Sensor Nets. Here’s a paper where they discuss their comparison of 5K vs 40K impedance differences in the EEG signal. No significant differences were found.
This is because modern EEG amplifiers have very high input impedances, hundreds of megohms to 1 gigaohm. The ADS1299 used by OpenBCI Cyton is 1 gigaohm. Only if you have a very old amp with impedance less than 100 or 10 megohms, would you need to be concerned about precise impedance. The old recommended standard for electrode impedance was 5K ohm. That no longer is the case with modern amps.
Some academic publishing may still require the 5K standard, but in practice that standard is now less justified. On the plus side, doing away with the 5K requirement means that the skin no longer needs to be be extensively prepared, as in the past with abrasives. Less potential for contamination.
This information is very useful for me.