I’m quite confused about this: Bipolar measurement is measuring one electrode signal with respect to the other electrode. In other words it is giving us the difference (signal(electrode1) - signal (electrode2) . Does this mean that all sorts of noise would be directly eliminated without having to use the GND electrode ?
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Insaf, hi.
The Ground electrode in EEG or ECG has TWO functions: primarily it acts as a way to center the differential amplifiers used on each channel (between the measured site and reference.) This helps the electronics to compensate for any DC offsets present.
Secondarily, the ‘Ground’ electrode is also sometimes referred to as ‘Bias’, because it is used to inject a small voltage back into the subject, that is 180 degrees out of phase with the common mode noise present on the channel / reference pair. This helps to further cancel out the common mode noise. Which is primarily from mains currents in the area.
Regards,
William
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Hi wiliams,
Thank you so much for your answer.
There is just one little thing that would like to confirm: Is signal completely drowned in noise when we don’t use the GND electrode?
I’m trying to capture alpha waves from occipital region using only two electrodes. Is that even possible?
Thanks again.
Cheers!
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Hi Insaf,
I would not omit the ground entirely. If you must, just connect it to one of your two bipolar leads. So you still have just two electrodes, but one of them has ground.
In EEG that uses a cap or headset, it’s fairly common to place the ground / bias lead on the midline of the 10-20 map. Electro-cap places it at AFz. The idea being that the counter common mode injection will then reach both hemispheres more evenly.
What exact manufacturer / model / of EEG amplifier (or chip) are you using?
Regards,
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Hello,
I’ve tried three types of electrodes: Cognonics dry flex sensors, Foc.us dry electrodes, and a pair of electrodes. I’ve been using only two electrodes from each type. The electrodes output go into a differential analog input of an ADS1298 which I’m connecting to a Raspberrypi for signal processing.
I’m only using the differential input (only two electrodes) But I’m also using a filter in my signal processing code that is supposed to cancel out the 50 Hz component. Isn’t that enough ?
Thanks again,
Best regards,
Insaf.
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I thought the foc.us dry electrodes were active, is that true? Is it documented how to drive these with the supply voltage and receive the output of the active amplifier? Is the output scaled at all? An active electrode would not be suitable for a ground connection.
Your overall impression of the foc.us dry? My read from their website is that their EEG product is struggling with few customers.
Would also be interested in your review of the passive Cognionics.
If omitting the ground is working for you, then you’re probably ok. For the passive electrodes I did suggest just tying ground to one of the diff pins, should give you some common mode injection advantage.
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I am using ADS1299 for experiment. I am confused with Bias, Ground and reference pin. The amplifier used in it is differential that means it measures one input wrt another. I understand the importance of Gnd to be connected on body so that our body also has same reference potential as the circuit. But according to openBCI (EEG Hacker: OpenBCI: Measuring Electrode Impedance) the BIAS pin will itself act as a driven ground and it didn’t use ground pin. Please help in getting the concept of bias and ground clear. They both are different I am sure of that.
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Hi Swapnill,
ADS1299 uses the Bias pin as BOTH Ground (centering the amps), AND as an inverse phase mains injection, to counter some of the mains noise. It can do both these functions at once, it is not an either or situation. The SRB2 / reference pin is separate from Bias, and is the differential reference source for the EEG channels.
William
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