Difference between Electrode impedance and Skin-Electrode impedance?
I am learning about electrode impedance and have a couple of questions.
There are two terms used frequently, Electrode impedance and Skin-Electrode impedance. Do these refer to different kinds of impedance or are the terms used interchangeably?
There is an outdated recommendation that there should be less than 5 KOhms impedance. The Ten 20 paste apparently has an impedance of 40 KOhms, and other pastes would probably have a similar impedance, so how was it possible to get a Skin-Electrode impedance of less than 5 KOhms?
Comments
Hi Silver,
In biosensing applications, the most important metric is the skin-electrode impedance. And this is what Cyton and Ganglion report in the GUI and APIs. In general when you see the shorter term "electrode impedance" it ALSO is referring to the skin impedance unless further qualified.
The document link you refer to:
http://wwe.eeginfo.com/research/ElectrodeImpedance.pdf
This research paper is NOT outdated, but refers to the research at EGI to determine what skin impedances are acceptable in newer amplifiers having higher amplifier channel input impedances. The Cyton input impedance is 1 gigaohm. So as you can see even much higher than the figures cited in the paper.
I'm not sure where you are getting the Ten20 paste 'impedance' you cite. If an electrolyte or electrode-only impedance is mentioned, it is usually qualified by the equipment and setup used to measure it. For example a square centimeter of paste between two plates, etc. This is not what is happening when the paste is used on skin with electrodes.
5K ohms skin impedance is indeed a standard used in much research prior to modern amps. My guess is that with evidence such as the EGI paper, modern research publications would be satisfied with impedances in the range of 5K to say 20K. Provided an electrolyte of some sort was used, and amp has decently high input impedance, say hundreds of megohms to gigaohm.
William
Thank you, William.
Few questions to clarify the details.
Is the impedance of an electrode checked with respect to another electrode? Or is the whole circuit impedance divided by two? Which one of the three combinations do the Cyton and Ganglion use to calculate the impedance?
"The EL-CHECK permits simultaneous connection of up to three electrode leads, for quick impedance checking between any two electrodes in the three connected leads Active (Vin+), Reference (Vin-), and Ground (GND)."
https://www.biopac.com/product/electrode-impedance-checker/
"In order to prevent signal distortions impedances at each electrode contact with the scalp should all be bellow 5 K Ohms, and balanced within 1 K Ohm of each other." https://www.measurement.sk/2002/S2/Teplan.pdf
Why do the impedances need to be balanced? Which of the three types of electrodes need to be balanced within a certain range of each other?
https://eeghacker.blogspot.com/2014/04/openbci-measuring-electrode-impedance.html
https://eeghacker.blogspot.com/2014/04/impedance-of-electrodes-on-my-head.html
https://www.google.com/search?as_q=ads1299+impedance&as_sitesearch=openbci.com
re: 1K ohm balancing.
Generally if all your electrodes are BELOW the chosen threshold impedance, there is no need to balance all of them within 1K ohms range. This is excessively obsessive.
I have seen similar electrode-impedance matching recommendations. What is the reasoning behind it?
https://www.quora.com/How-bad-can-an-EEG-get-if-electrode-impedances-are-15kohms-vs-5kohms-What-is-an-acceptable-impedance-difference-between-all-electrodes
Does electrode-impedance(or electrode-impedance differences) also have any phase distortion effects on the signals?
Please re-read,
http://wwe.eeginfo.com/research/ElectrodeImpedance.pdf
Cyton input impedance is 1 GIGAohm.