Ganglion Impedance measurement
Hello,
I was looking at the schematics of the Ganglion board and am a little bit confused about the impedance measurement part. From what I understand, impedance measurement is used to check the impedance of the electrode and skin contact.
The schematic shows a DAC outputting a voltage which is fed to the IN+ pin of the Diff OpAmp through a digital switch. Also this signal is superimposed on the REF pin through another digital switch. I presume while doing a impedance check the corresponding DIP switches are placed in IN- position. How is the impedance calculated ? Is it a ratio of the DAC signal to the signal measured by the ADC. Is the type of the waveform used specified in the documentation.
Cheers
Thomas
Comments
Hi Verve,
Here is some background on how current injection can be used to measure impedance, on the ADS1299 (Cyton) chip.
https://eeghacker.blogspot.com/2014/04/openbci-measuring-electrode-impedance.html
https://eeghacker.blogspot.com/2014/04/impedance-of-electrodes-on-my-head.html
Ganglion does a similar current injection, generated by the DAC. The signal injected is usually a sine wave within the band of EEG frequencies. In the case of Ganglion this is being generated by the DAC driven from the Simblee code.
https://github.com/OpenBCI/OpenBCI_Ganglion_Library/blob/master/OpenBCI_Ganglion_Library.cpp#L390
William
Hi William,
Thanks for the reply.
Ganglion does a similar current injection, generated by the DAC.
From what I can see, it sends a voltage and not a constant current from the DAC. Also I think you mentioned in of your older posts that it is a triangular voltage and not a sine wave. Just out of curiousity, why use a sine wave or a triangular wave ? Can it not be plain DC ?
If you examine the multiple sheet schematic,
https://github.com/OpenBCI/Ganglion_Hardware_Design_Files/blob/master/Ganglion_SCH.pdf
On sheet 2 is shown an output "micro-amp sense" line. All that is needed to use ohm's law, E = IR, or R = E/I, is to have known values for the voltage and current. So apparently Ganglion uses the constant voltage source and derives the current flowing.
re: triangle vs sine, I gave the code link earlier. I don't recall saying earlier that it was a triangle, but it could be. I assumed they were using some sort of table lookup for the sine wave quarter segments. But they could just be doing linear interpolation.
It's not that critical which is used, as long as it is AC. You cannot use DC for skin impedance because that interface / electrolyte would polarize over time and not give accurate reading. On Cyton a specific AC sine injection frequency is used and this makes it easier to filter out any noise components from environmental effects happening in the room.
Thanks much for your valuable time and advice.
On sheet 2 is shown an output "micro-amp sense" line.
I will simulate the circuit using LTSPICE and check and post back.
It's not that critical which is used, as long as it is AC. You cannot use DC for skin impedance because that interface / electrolyte would polarize over time and not give accurate reading.
Never thought of that. I was wondering why not just use a DMM and just measure the resistance... By AC, I think you mean with a DC offset and not going in the negative direction ?
No, no DC offset. DMM will not work for EEG impedance. There are special meters for practitioners with older amps.
https://www.google.com/search?q=eeg+impedance+meter
Actual AC must be used to avoid the polarization aspect. A DC offset would polarize. The resulting output measurement is just rectified and averaged.
If you look at the source code you can see that they also verified the accuracy of the impedance values by doing tests with real resistors. Then adjusted scaling factors.
Unrelated to this discussion, I saw this link you had posted .
http://www.eeginfo-neurofeedback.fr/fileadmin/images/was_ist_neurofeedback/wie_funktionierts/SCP_recording_electrodes_and_paste.pdf
Unfortunately its gone dead. Any chance if you still have it , would appreciate.
Cheers !
You can download it here:
https://www.researchgate.net/publication/7942803_Tallgren_P_Vanhatalo_S_Kaila_K_Voipio_JEvaluation_of_commercially_available_electrodes_and_gels_for_recording_of_slow_EEG_potentials_Clin_Neurophysiol_116799-806
_Actual AC must be used to avoid the polarization aspect. _
Could you explain further, please ? I am a bit confused , because there is no negative potential on the Ganglion board. In the Cyton board we have a voltage inverter to get a negative potential.
AC is ALWAYS used to measure EEG impedance. Have said this multiple time now. All that matters is the RELATIVE voltage differential. Not absolute voltages. What is your goal with Ganglion, have you been able to use it successfully?