I have been playing with a four-channel EEG neurofeedback device. There is some evidence, not conclusive but respectable, that coding performance is improved by neurofeedback on the ratio of beta power over theta power. I would like to see if that is true for me.
My question concerns cable shielding or other noise reduction techniques. I work in an old building with two-wire electricity. So there is no accessible quality ground, and we had to shell out some extra money for extra-high-end surge protectors as a result. And there is a lot of electrical noise around my work station — fluorescent lights, a laptop, desktop, and server, printer, several screens, plus miscellaneous other equipment, all within six feet of my head The result is a much noisier signal than I get using the same equipment at home.
The EEG is battery powered, so there is no direct coupling to the building power supply. The output goes to my computer via USB cable, so there might be some noise from there, but the same is true at home. So it is the incremental noise, induced by office equipment in either the leads to my head or the USB cable, that I want to shield against.
Is there a good way to do that? I would try grounded coaxial cable, but I don't have a ground.
Best Answer
The Electric Field or E-field of high impedance noise can easily exceed 50 V/m from a nearby power line with 240Vrms or ~679Vpp. But at this high impedance fento-farads ) and high body dielectric constant there is very little current thru our body at 50 or 60Hz, and low voltage drop, since our body is many orders of magnitude lower impedance than the stray E-field coupling impedance C ( fento-farads ).
What should be done?
We need to use very high impedance differential amplifiers with very high COmmon Mode Rejection Ratio CMRR such as the ANA116
Consider high quality EEG's can achieve a low noise in the range of 1uV , that's a ratio of 8 decades or 160 dB for the best performing instruments or a CMMR of 160dB which is far greater than what an INA can do unless you use every method suggested in INstrument Amplifier (INA) Application Notes. (AN #)
- You can make your own with the average of all electrodes and buffer that to create an active "virtual ground" using the +Gd and -Gd (guards) on the INA116
However even "Guarding" has it's limits if the CM input exceeds the linear input range.
What has been tried?
How can that fail?
What is the most common practice?
What else is possible? - using non-gel coupled electrodes at 0.1mm gap, with center pad and guard ring to measure the near-proximity electric field with active INA above round PCB Electrode
What is a good battery pair to use that are low impedance and stable voltage?
six 16850 Lipo 3.6V. rechargeable ( more initial cost but longer useful life after few hundred cycles)
Other notes