EEG over 1000uV values even after filters apply [resolved]
Prageethda
Colombo, Sri Lanka
I use cyton as Hardware for acquire the eeg data and brainflow library(for python) as the software. the method get_current_board_data (256) is used for acquire the latest 256 data points of buffer.
after acquire the raw data and it is multiplied by SCALE_FACTOR_EEG = (4500000)/24/(2**23-1) #uV/count
for convert it to microvolt values. but the signal is a oscillation over 1000uV values.
then according to the https://openbci.com/forum/index.php?p=/discussion/201/large-millivolt-data-values-fbeeg-full-band-eeg post the data are band passed(cutoff =10Hz) . but it is still the signal like a sine wave. and values oscillating over 1000uV.
Then i used the > OpenBCI GUI and apply filter in it . the GUI also showing values oscillation over 1000 uV. normally eeg values are in 0 - 100uV . what can be the problem ?.
Comments
Why can be the reason for this kind of high impedance.? Is there any method to configure this?
Prageethda, hi.
You did not state what electrodes you are using, nor your Reference and Ground/Bias location. What happens when you follow the steps exactly, in the Cyton and EEG tutorials? The recently added 'Gain Conv' menu, should be set to 'Body uV', don't use 'Classic', that inflates the readings. This menu will be removed on the next release and the default will be Body uV.
https://docs.openbci.com/docs/01GettingStarted/01-Boards/CytonGS
https://docs.openbci.com/docs/01GettingStarted/02-Biosensing-Setups/EEGSetup
It seems likely that things may work normally with the GUI, but fail with your Python program. If that is the case, you can ask questions on the Brainflow Slack,
https://brainflow.org/
If you happen to be using the dry electrodes, this thread discusses expected impedances. Yours are obviously too high even for dry electrodes.
https://openbci.com/forum/index.php?p=/discussion/2517/expected-impedance-range-with-ultracortex-cyton-resolved
There are also many threads discussing the RAILED condition, and how to address it.
https://www.google.com/search?as_q=railed&as_sitesearch=openbci.com
William
Thank you for your reply.
I followed exact same methods I use gold cup electrodes and ear lobs are used as reference and bias location. When is set the Gain conv to 'Body uV', I got the acceptable readings. Can you suggest a post for explaining what are the difference in Gain Conv methods(classic and body) ?
Your impedances are still WAY over where they should be. Gold cups and Ten20 paste should produce impedances even below 50K ohms, as low as 5K ohms. Your screenshots show 8 MEG ohms.