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Fix Cyton impedance check board configuration #1211

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Fix Cyton impedance check board configuration #1211

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0a99b44
Merge pull request #1192 from OpenBCI/development
retiutut Sep 22, 2023
d8543aa
Merge pull request #1197 from OpenBCI/development
retiutut Sep 28, 2023
b3918c8
Fix Cyton impedance check board configuration
philippitts Feb 29, 2024
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4 changes: 2 additions & 2 deletions OpenBCI_GUI/BoardCyton.pde
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Original file line number Diff line number Diff line change
Expand Up @@ -450,8 +450,8 @@ implements ImpedanceSettingsBoard, AccelerometerCapableBoard, AnalogCapableBoard

currentADS1299Settings.values.gain[channel] = Gain.X1;
currentADS1299Settings.values.inputType[channel] = InputType.NORMAL;
currentADS1299Settings.values.bias[channel] = Bias.INCLUDE;
currentADS1299Settings.values.srb2[channel] = Srb2.DISCONNECT;
currentADS1299Settings.values.bias[channel] = Bias.NO_INCLUDE;
currentADS1299Settings.values.srb2[channel] = Srb2.CONNECT;
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https://docs.openbci.com/GettingStarted/Biosensing-Setups/EMGSetup/#obtain-emg-data-with-openbci-cyton-board

Streaming EMG Data with the OpenBCI GUI

Once you have the GUI open, turn off any channels that you have not connected to muscle sensors.

Then for Cyton, go to hardware settings and turn off SRB2 for all of the channels that you are streaming data from.

Note: This is because ordinarily SRB2 is the reference point from which potentials are measured. However we are now having muscles reference themselves, so we do not want it to look at this pin.

While this may work, it also seems like this may break checking impedances when doing EMG.

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Relates to #1058

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@philippitts philippitts Apr 19, 2024 •
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It does not matter if you are reading EMG or EEG. The impedance check remains the same and measures the impedance of one Cyton electrode against SRB2, regardless of where that electrode is placed. Per the tests above, BIAS should never be used for impedance checks. This makes sense because BIAS is used as input to the ADS1299 to create destructive interference patterns that reduce noise vs SRB2 which is the reference point against which a channel can be measured (usually for EEG).

If you look at my readings from the linked issue you can see that on almost all the BIAS tests the signal was steadily dropping. This is because the ADS1299 is canceling out the "common noise" it's reading from the BIAS pin.

EMG doesn't use SRB2 while acquiring signals because EMG references against a second EMG channel (in most cases), but to measure impedance, SRB2 must still be used. The easiest method is to switch the BIAS electrode to SRB2 for impedance measurements and switch it back to BIAS for signal acquisition.

currentADS1299Settings.values.srb1[channel] = Srb1.DISCONNECT;

fullCommand.append(currentADS1299Settings.getValuesString(channel, currentADS1299Settings.values));
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