Channel Setting Command Definitions
FiveFunnyThree
Berlin
in Cyton
1.) In here you define the input type values: ADSINPUT_NORMAL, ADSINPUT_SHORTED and so on. Is the behavior of those types defined in the ADS1299 data sheet, or where?
2.) in BIAS_SET, we select to include the channel input in BIAS generation. Does channel mean Px or Nx, or is it already Px-Nx or some other behavior?
3.) In SRB1_SET is there a SRB1-Pin available?
I try to understand the interactions of Px,Nx, SRB1/2 and BIAS on the board, which is quite customizable on AS1299. I always get the least fluctuating results if I pin the current of A1 clip to both SRB2 and BIAS in parallel.
Comments
re: ADS1299 test configurations. Not all of these combinations are available, but some can be selected with the SDK commands:
https://docs.openbci.com/Cyton/CytonSDK/#test-signal-control-commands
re: Bias / SRB2. Have you read the tutorials?
https://docs.openbci.com/GettingStarted/Biosensing-Setups/EEGSetup/
Normally for EEG, all the INxN pins are used, these are the pins closest to the board surface. If you then use the SRB2 as reference (SRB pin closest to board), this is bussed to all the INxP pins. Thus acts as a common reference for all EEG channels on the INxN pins.
Bias is the same as Ground in other EEG systems and should NOT be connected to SRB2, but instead placed as a separate scalp location. A common practice is to place the ground on the midline, at say AFz or POz. This enables the 'Bias' function of the ground lead, to cancel some of the mains EMF interference.
William
See page 50 of the data sheet for the Channel Input mux definitions.
The state of the BIAS_SET, if on means include this channel in the Bias 'calculation'. Off means exclude. When doing mixed mode EEG EMG ECG, Bias should be turned off for EMG and ECG channels, since those signals are of such high amplitude, that they would swamp any anti-mains action for the EEG channels. And would 'bleed' into EEG channels if turned on.
Yes I read and tested the standard electrodes to pin placement as explained on your homepage. But regarding the BIAS, lets assume I want to do a few differential Pick-ups, say F7 - T5 and at the same time F8-T6 (or whatever). Then I would for example pin F7 to P1, T5 to N1, F8 to P2 and T6 to N2. And then I would still pin say A1 ear clip or Cz to BIAS?
You should DISCONNECT channels 1 and 2 in that case, from the SRB bus. And probably from Bias as well. You will continue to use A1 for SRB2 and Cz as Bias for the referential channels 3 to 8.
I see.
So if I deactivate 3-8, then A1 and Cz are not necessary right?.
Remember that Bias also acts as Ground, and helps the differential amps to center themselves. There must always be a Ground connection with any EEG amp. So even though your wired channels 1 and 2 as self referential, Ground is still needed. Cz is a good position, or you could use the closer ear lobe.