Impedance question - noisy EEG signal
stygiangreen
Darmstadt, Germany
Hey guys,
I'm measuring impedances using the Cyton Signal widget in the GUI.
However, when no pin is connected I am getting non-sensical impedance values around 3 kOhms.
I read that they are supposed to be as high as 5000 kOhms in open circuit (https://openbci.com/forum/index.php?p=/discussion/3726/electrode-impedance-test-using-the-gui-widget).
As for the hardware, I am using a third party pre-built solution containing a cyton, a daisy, a wifi shield and a battery board.
Can anybody help explain this behaviour? My main problem is that I can't get anything more than noise with both passive and active EEG electrodes. So I'm in the process of outruling internal problems with the hardware.
Impedance values:
My hardware:
my noisy EEG signal using passive (channel 1) and active (channel 6 and 7) electrodes:
Comments
Hi Stygian,
Unfortunately the system you purchased is NOT manufactured by OpenBCI, but instead by a unethical Chinese 'clone' company. There are other threads on this forum by unsatisfied users of this clone system. For one thing, the OpenBCI Wifi Shield has issues when paired with the Cyton, the noise pulses you show on your channel 1 plot above correspond to this noise issue. I believe your channels 6 and 7 also show these same noise pulses. The Shield worked fine when paired with Ganglion. Hence one conclusion is that firmware mods are needed in the Cyton - Shield coordination.
Because of this issue, the Wifi Shield was removed from the Shop until engineering changes are derived. Unfortunately the unethical Chinese company decided to push ahead and sell this compromised configuration with no firmware fixes. I suggest you try to get a refund if possible from the seller.
Regarding the impedance question, it is never recommended to use channels that are unterminated at the subject. You need all three leads connected: channel, reference, and ground (Bias). Otherwise you will pickup environmental noise. One common test scenarios connects these three leads together with a blob of paste or alligator clips. This should then produce close to zero ohms impedance, verifying the readings.
Sorry to convey the bad news.
William
You might try experimenting with the mode in which the Shield is connected to your computer: Wifi Direct, vs via your router. May influence the noise spikes.
A fix for the impedance issue is forthcoming shortly. Sorry for the delay in resolving this.
https://openbci.com/forum/index.php?p=/discussion/3790/high-impedance#latest
William