Capacitance feedback loop on contact (with earth)

eeg
Tags: #<Tag:0x00007f460316ad00>
#1

Can anyone share experience on (intermittent) capacitance interference using an EEG headset? I include a video here. I am running a wireless headset with ‘perfect’ electrode contact and standard HAMMING filters. I think this might relate to a low voltage leak but I am confused by the intermittent phenomena (which only occurs on the DC side occasionally). I am wondering if it has anything to do with inverting polarisation?

#2

Hi, Neuro,

Many laptops when plugged into AC with their TWO prong power cords – are seriously contaminated with large amounts of mains noise. Getting your hands anywhere near the case or frame of these computer, will induce large mains noise in your EEG signal.

One solution is to use a cord like this,

http://www.lessemf.com/computer.html#295-usb

It grounds the frame / case of the laptop and removes the mains noise. Be sure to verify your ground pin in the wall socket is a real ground. This can be done with an outlet checker.

Regards,

William

#3

Thank you for your attempt. It was also a consideration of mine. I have tested the issue with regards to mains noise and can indicate that this phenomena is also true when the device is not connected mains supply. I mentioned in my first post that this might relate to the body (and its electrical capacity). I have included another video showing a similar response when the body is connected to a good earth (but the unit is not). Therefore, this is not a mains issue but an earth interference issue (which I mentioned might be acting as a circuit). The new video is available here: https://cp.sync.com/dl/a5838daa0/6x9526vy-xs55zf57-6y96v5td-6pvfa3pw

#4

Can anyone else help on the DC side of this problem?

#5

Mains noise is ubiquitous. It’s present around every conduit in your home containing the wiring. Which runs under floors, through walls, through ceilings. All AC to DC power supplies also radiate mains noise, or even higher frequencies if they are switching based power supplies. These EMF fields extend several feet in all directions around sources, and can be measured with EMF field meters. For EEG purposes you want to position yourself away from such sources.

Grounding either the body and/or the computer equipment will certainly help. Are you using a notch filter at the mains frequency? This is the normal way it is handled.

#6

Thank you again for your reply. I have purchased a mains tester and an EMF tester previously. I am not getting earth fault or any EMF reading on the tester unless I place this near to the computer CPU. To adjust the current view, my second test was testing the earth with dedicated contact (hand) direct to earth. I have found that the stronger the earth connection the greater the interference pattern. A more detailed view of the montage is available here.

This contradicts what I think is being conveyed about mains noise. I have a few more tests to do on the voltage side, but I will keep this PM alive to see if there is any experience of this type of polarised interference. What I think is happening is that the ‘body electric’ is being grounded and hence a current is being drawn to the contact point from a source of capacitance. In other words, my testing shows that earth is in fact acting as a slight negative thus amplifying the sensor pick-up. To remind the responders I am running on battery without any connection to either DC or AC on the headset side and using mains earth on the (hand) contact side.

I have also found a gyroscopic effect relevant to this capacitance and here EMF appears relevant. I will post elsewhere on this topic as I think this is a separate subject and I don’t want to confuse the issue of earth.

#7

The best strategy with EMF fields is simply to position your EEG AWAY from any power cords, wall warts, conduits (floor, ceiling, walls), routers, cellphones, etc. Earthing can change some of the amplitudes of these sources of interference, but is not a complete solution.

#8

The best my EMF monitor can do is 0.0 micro-Tesla. This is the result I get from the general test space. Only the CPU and power sources themselves show up as any readable value. I do however have indication of a 0.2 micro-Tesla value at a height of around 2m which might relate to cabling. However, my cable tester has not shown any uniform cable plan at that height - so I am still bemused about this small value. Regardless, the test space is typically below a readable level. This is why I moved on into micro-voltages instead, as I have found graphical fluctuations in voltage when the device is being used suggesting poor regulation of connectivity. I.e. I am quite confident this relates to conduction.

#9

EMF meters have a wide range in price, quality and sensitivity. What is the manufacturer and model of your meter? Cost? The better meters start in the hundreds of dollars range, and the lab quality ones, well above that.

#10

Thanks for the suggestions, I am yet to calibrate against sources. The meter matches the sensors on my smartphones as well, so I don’t expect much error for indefinite sources. Obviously, the amount of EMF is an important factor. As mentioned, there is no constant source in the test space. I have also tested this outside to ensure mains noise is not an issue. I have isolated it down to the headset itself and obviously the body contact with earth.

Mains noise is not the answer to this PM.

#11

To expand the consideration of touch connectivity I want to introduce the plausibility of (a known and common effect called) virtual capacitance. Please share any experience of virtual capacitance (which appears coincident with the) associated resistance differentials, and here I am thinking of forced division of electro-statics (i.e. virtual capacitance and their effect on the polarisation on multi-sensor networks). In short, the use of body impendence to generate a differential above the natural brain wave (when connected to relative earths).

#13

In other words this may support the notion of artificially induced impedance due to virtual capacitance (or some other means by which voltages are divided by a virtual ground).