Cyton Impedance Measurement, at different frequencies?

svengisler · August 2019

It is my intention to measure the skin impedance of electrodes with the Cyton board. Normally this is done at 8Hz or 10Hz for EEG but I would like to measure with a certain frequency range.

Is it possible change the frequency of the impedance measurement (between 8Hz and 125Hz or even higher) with the Cyton board?

Thank you for your answers.

wjcroft · August 2019

Sven, hi.

The ADS1299 is only capable of injecting at certain frequencies, 31.2 Hz, and 7.8 Hz, see page 31.

http://www.ti.com/lit/ds/symlink/ads1299.pdf

The Cyton uses the 31.2 by default.

In actuality, the ADS has a VERY high input impedance: 1 gigaohm. With such a high input impedance, skin impedance is much less of a concern, see,

http://wwe.eeginfo.com/research/ElectrodeImpedance.pdf

Regards,

William

Billh · August 2019

see chart page 651 at https://www.ahajournals.org/doi/pdf/10.1161/01.CIR.34.4.649

faheemersh · January 2020

@wjcroft How can we determine what frequency (31.2 or 7.8 Hz) is used during the test? Also, is there any circuit diagram showing how the ADS1299 on the cyton performs the skin to electrode impedance measurement? I checked the manual and the page you mentioned and I saw some diagrams on the pages before and after 31, but not sure if those diagrams are relevant.

wjcroft · January 2020

It's 31.2 Hz.

wjcroft · January 2020

See these posts by Chip for background on the injection method of impedance measurement,

http://eeghacker.blogspot.com/2014/04/openbci-measuring-electrode-impedance.html
http://eeghacker.blogspot.com/2014/04/impedance-of-electrodes-on-my-head.html

Also look through other links on this thread,

https://openbci.com/forum/index.php?p=/discussion/165/measuring-electrode-impedance

And,

http://openbci.com/forum/index.php?p=/discussion/84/ads1299-electrode-impedance-measurement-algorithm

Cyton Impedance Measurement, at different frequencies?

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