How Cyton amplifies EEG with gain of 24x

mercurial

Hi,
Just trying to wrap my head over some points
I see that the Cyton chip (ADS1299) has a max gain of 24.
Considering an EEG signal around 20uV input we would get a 480uV or ~ 0.5mV output.
Also the ADC reference on the ADS1299 on the Cyton are set to
Vref+ = 4.5V
Vref- = -2.5V
Which gives a total range of 7V.
So 0.5mV is almost 0.5/7000 * 100 = 0.007 % of full scale
Considering usable bits as max 20bit for 24x gain and 250sps
the resolution would be 0.007/100 * 2^20 which is ~ 6bits of resolution.

Is this ok for EEG, or do we need to use an external amplifier to increase the gain, also an external reference of maybe 1.2V or lower, to get more resolution out of the board?

wjcroft

Hi, Mercurial,

No, there is plenty of resolution, see this page,

https://docs.openbci.com/Cyton/CytonDataFormat/#interpreting-the-eeg-data

For the scale factor, this is the multiplier that you use to convert the EEG values from “counts” (the int32 number that you parse from the binary stream) into scientific units like “volts”. By default, our Arduino sketch running on the OpenBCI board sets the ADS1299 chip to its maximum gain (24x), which results in a scale factor of 0.02235 microVolts per count.

Actually for EEG purposes, sub-microvolt samples are most likely pure noise. Lowest practical EEG sample would be 1 microvolt.

William