Is there a short in my Cyton?
stuffdeveloper
Indianapolis, IN
in Cyton
In the figures above. I have an electrode connected in just N2P (top fig - purple line) and an ear clip in SRB2 (bottom fig). With nothing connected to the reference there is some bleed through in all pins from my tapping on the electrode connected in N2P (electrode 2 - the purple line). With a ground connected (SRB2), I see strong bleed through into pins 1 and 3 only. This is some odd behavior. Is there some short or across pin coupling happening in my cyton?
Comments
All EEG, ECG, EMG, is connected to a differential amplifier. This subtracts the two signals. So both the channel electrode AND the reference electrode (SRB2 for EEG), must be connected to the subject. Free floating electrodes or reference just generate noise.
You also must have the 'Ground' (same as 'Bias') in all measurement scenarios.
So the ground for the measurement is the 'Bias' pin and then each electrode pin is differentially amplified relative to SRB2? This type of info doesn't seem clearly laid out in the docs or am I just missing it?
Have you looked at the tutorials?
https://docs.openbci.com/GettingStarted/Boards/CytonGS/#v-connect-yourself-to-openbci
https://docs.openbci.com/GettingStarted/Biosensing-Setups/EEGSetup/
Yep, that explains how to setup up but not the why on things are the way they are. Say I wanted to recreate a personal version of the cyton that is more tailored to my application, its useful to know these details I think
All biosignal amplifiers share these characteristics: differential inputs, common reference available (in case of EEG), ground.
https://www.google.com/search?q=eeg+amplifier+inputs
ADC used in Cyton:
https://www.ti.com/product/ADS1299
See 'data sheet' document for how it is connected in circuit.
They also make a 'demo kit',
https://www.ti.com/tool/ADS1299EEGFE-PDK
I guess my question is more around whats happening on the hardware vs what the gui is showing. So is everything referenced to the 'bias' pin as the ground on hardware and the gui is just displaying everything referenced to SRB2? So the SRB channels are just extra channels being saved.
I posted some previous links with more technical backround.
No, this is not correct. When measuring EEG the reference is the SRB2 pin, which represents the ganged / bused pins IN1P, ..., IN8P (the top row of pins).
Bias / Ground is used in all biosignal applications to reduce noise and 'center' the differential amplifiers.
SRB2 is the reference both in hardware and software.
SRB2 is an electrical bus consisting of the the top pin inputs. EACH channel has two input pins, for channel 1 those are labeled IN1N (the normal connection to subject) and IN1P (the top pin, which in EEG is connected to SRB2).