raw data microvolts vs milliVolts
giuseppetx1
Italy
in Software
Dear all, sorry for the question, I know other posts already referred about the raw data interpratation.
As a check, I applied a 2mVpp sinusoidal signal from a signal generator to one of the PIN of the Cyton. With Gain = 1 the raw data report a corresponding sine wave acquired by the Cyton with an amplitude of 100. If the raw data are expressed in microVolts - as referred in the header of the raw OpenBCI file - it means 100uV, while I expected 2000uV.......what is going wrong? Thank you for your help...G.
Comments
Giuseppe, hi.
Do I understand correctly that you stated, 2 millivolts peak to peak? Which corresponds to +1 mV positive swing and -1 mV negative.
The default gain is 24, which results in (as the header states), microvolts being shown in the CSV file. But the CSV header does not change when you adjust the gain manually (sorry). The gain formula is shown in this page,
https://docs.openbci.com/docs/02Cyton/CytonDataFormat#interpreting-the-eeg-data
Normally with application for EEG, you just leave the default gain in place, then the values are shown in microvolts. Do please note that Cyton CSV file DOES NOT have any filters applied. So there will be a high DC offset, generally in the range of some millivolts, 10 millivolts is not uncommon. Your signal processing (DSP) will generally remove this by doing either a highpass at .5 Hz or a bandpass, say from .5 Hz to 45 Hz. Additionally a notch at your mains frequency (50 or 60 Hz) is helpful.
https://openbci.com/forum/index.php?p=/discussion/201/large-millivolt-data-values-fbeeg-full-band-eeg
Regards, William