Oddly High Apparent Low-pass Filter on Cyton Board


#1

Hi,
I have been designing an EEG device, and I am currently looking into the design of the analog filter stage before being fed into the ADC. I am only interested in high delta to alpha band signals, so I was intending to design a simple differential mode bandpass filter (my design is in bipolar montage) consisting of a ESD detection/prevention stage, a high-pass stage with cutoff frequency around 30 Hz, and a low-pass stage with cutoff frequency around 1Hz. I’ve been advised not to use common-mode shunt capacitors to reduce CMRR since common mode noise is-hopefully-not going to be a huge issue.

To sanity check, I referenced the openBCI Cyton board schematic and I was a little perplexed by what I found. As far as i can tell, the Cyton board is in referential montage and uses only an ESD stage and a common-mode low-pass filter. But that filter seems to have a ridiculously high cutoff frequency–if the resistors are 2.2kΩ and the capacitors are 1000pF, then the cutoff frequency must be f = 1/(2piR*C) = 72 kHz. This seems absurd. Am I missing something?

As a side note, any advice for my filter stage design would be very appreciated. Thanks!


#2

Kyle, hi.

OpenBCI Cyton is based on a delta sigma ADC. See this thread and other links,

The only function of the RC low pass filter is to knock out the holes in the sinc filter that is built into the ADS1299. See the TI data sheet. The RC also suppresses some of the potential EMI that might come in from nearby RF sources.

As far as your own EEG, my suggestion would be to do ALL signal processing / filtering in software. That gives maximum flexibility to your design. A related post,

Regards,

William