Cyton RC pre-filter values

taylorwebb85

Can some one point me to documentation on any filtering happening on the board? I am guessing there is some anti-aliasing filtering before the signal is passed onto the PC? I am using the Cyton V 3-32.

wjcroft

No DSP filtering happens on the Cyton. The GUI recording file is raw, no filtering. But the GUI screen display filtering can be adjusted with the menus.

taylorwebb85

Right, thank you for this.
I am wondering about the characteristics of the anti-aliasing filter. There must be an analog filter before the sampling.

wjcroft

Taylor, hi.

Yes there is an RC filter applied to each channel before it reaches the ADS1299, required for sigma-delta ADCs. See schematic,

https://docs.openbci.com/Cyton/CytonSpecs/

2.2Kohms and 1nF. Here is an old thread on the subject,

https://openbci.com/forum/index.php?p=/discussion/154/front-end-filtering-esd-cmrr-rfi

William

stuffdeveloper

This would be a poor anti-aliasing filter for a fs=250 Hz, right? The cutoff frequency is > 7kHz

wjcroft

There is a large discussion on the RC values chosen at the thread mentioned in the previous comment.

https://openbci.com/forum/index.php?p=/discussion/154/front-end-filtering-esd-cmrr-rfi

Also see the ADS1299 data sheet for their recommendations,

https://www.ti.com/product/ADS1299