Cyton Board Low-Frequency Response Limit
Hi there,
I've been exploring the specifications of the OpenBCI Cyton Board and am curious about its low-frequency response capabilities. I noticed that the specifications for the Ganglion Board explicitly mention a cutoff below 0.3Hz, but I haven't found similar information for the Cyton Board. Can anyone point me to where, if at all, the low-frequency limit for the Cyton Board is documented? Any insights or guidance on this matter would be greatly appreciated, as understanding the full range of frequencies the Cyton can accurately record is crucial for my project.
Comments
Cyton is a "DC-coupled" EEG amplifier, meaning that there is essentially is no lower limit. See this thread.
https://openbci.com/forum/index.php?p=/discussion/201/large-millivolt-data-values-fbeeg-full-band-eeg
The infra-low frequencies below .5 or .1 Hz, are generally much higher amplitude than 1-40 Hz EEG. Which is why millivolt amplitudes are frequently seen below .5 Hz.
William
Hi William,
First of all, I would like to thank you for your prompt response to my query regarding the OpenBCI Cyton Board. Your information have been invaluable.
Now, I wanted to dig a little deeper into the concept of DC coupled EEG amplifiers, especially for someone like me with no engineering background. Your explanation clarified this issue for me, but I would love to have a concise breakdown to better understand it.
Could you give a simplified explanation of how the DC coupled EEG amplifier accomplishes this feat, touching on the differences from the Ganglion board, etc.? In addition, any beginner-friendly resources you would recommend for further reading on this topic would be greatly appreciated.
Did you read all of the material on the link I gave. That has excellent background.
Ganglion is NOT DC-coupled. There is a high-pass analog (RC) filter inside the front end. I believe it blocks EEG below .5 Hz. See the docs or schematics. Cyton's ADS1299 was designed specifically to be DC-coupled.
In the old old days of EEG amps, they were termed "AC-coupled" because there was no circuit path to get the DC / very low frequency signal through the amp. Typically because of using capacitors for coupling, which block DC.
Thank you for your prompt reply and for providing detailed explanations.
I now understand that the Ganglion Board incorporates a bandpass filter that cuts out any frequency below 0.3 Hz, a detail I found in the OpenBCI documentation.
Furthermore, your explanations about the AC coupling mechanism and the features of the Cyton Board were enlightening. It's now clear to me that the Cyton Board is DC-coupled and capable of accurately transmitting signals across a wide frequency range.
Thanks, yes the Ganglion is using a high-pass RC filter. It is not a 'bandpass' filter. It does not limit the upper end of the EEG spectrum. They made that decision in part because the original Ganglion packet format used a 'compression-algorithm' to encode the ADC values. Including the millivolt level DC offset, would have confused the compression technique they had.
The current firmware revision of the Ganglion packet format no longer uses compression.
Your clarification on the Cyton Board being DC-coupled and the Ganglion Board incorporating a high-pass RC filter, not a bandpass filter, has provided me with a clearer understanding. Additionally, learning about the AC coupling mechanism and the features of the Cyton Board has been enlightening.
Thank you once again for taking the time to address my queries and for sharing your expertise on this matter.
Otemo