EEG part 3: Different passive electrode types

Some notes first:

See http://openeeg.sourceforge.net/doc/hw/electrodes/passive/ for inspiration. From my experience here, maintaining a good and continuous contact is more important than actual materials. A discussion of elastic bands to secure electrodes would be useful.

All of these tests were repeated with different levels of skin preparation (basically before and after rubbing sandpaper and alcohol on) and electrode jelly/saline. For a quick test, getting ECG signals while holding the electrodes between the finger and thumb of each hand is the most convenient

Electrode Tests

Initially, these tests were done with electrodes made with meter long RG58 coax soldered to ‘silver’ disks I ordered off etsy. In retrospect, they were obviously not silver. I secured the disks to the insides of my forearms with rubber bands and then typed commands while trying not to mess that up too much. This was an issue, as I would like a wearable setup and RG58 is just too thick and stiff not to shove the electrodes around causing large jumps in potential. I’ve since tried other electrode types.

Pogo Pins

Pogo pins are small, spring loaded spears with resistances of around 1 Ohm. They are used normally as surface probes for automated testing of assembled PBCs. Because they are spring loaded, I imagined the physical contact might be more secure than a flat disk on the skin.

Pogo pin electrode, no cable attached yet

I assembled the array of pogo pins by stacking layers of protoboard to keep them aligned and then soldering them in. While they worked, they did not work as well as the silver disks. The pressure it took to maintain good connections left marks.

Gold Electrodes

One point of annoyance is that chemical effects can create large potentials compared to ECG signals. For my purposes this could be filtered out with a high pass filter, but I would prefer not to do this. Being relatively chemically inert, gold is a decent material for electrodes (see Evaluation of commercially available electrodes and gels… Clinical Neurophysiology Tallgren et al 2004 for more). Sintered silver chloride is understood to be the best, see the next section for that.

Cheap gold plated electrode. The cable is very flexible which is good for preventing the tugging which causes movement artifacts.

I tried some gold plated electrodes from ebay. I would have loved to use the included cables but well shielded coax has really proved nessesary to prevent strong 60Hz interference [see footnote at end]. These are better than the silver disks, but still not ideal.

Disposable Sintered Ag|AgCl

Again off ebay I ordered electrodes, but this time disposable sintered Ag|AgCl ones.

The silver patchs on the ends of the coax are the undersides of used electrodes.

These are already coated with conductive adhesive so they stick to skin well. In an actual ECG, these are grasped with clips on the white tab, which has a conductive coating underneath it. I tried both using aligator clips on the tab and just sticking the wire underneath and the latter worked better. It is possible a different type of clip would be an improvement. This did not seem to be an improvement upon the gold electrodes, so I decided to stick with them.

*Most articles on ECG and power line noise emphasize the need for strong common mode rejection-CMRR is not the issue here. The issue is the differential signal which can be demonstrated by holding the wires out at different angles. When the wires are parallel and near to eachother, all the powerline noise is common mode and there is no 60Hz signal. When they are held out in opposite directions, the 60Hz noise is maximum, corresponding to a large differential 60Hz pickup. The obvious solution is to have wires take similar paths and only break apart at the end. I have tried this with dual conductor cables where there is only ~12″ of difference and the interference is still worse than with good coax.

Differential 60Hz signal still significant fraction of the total

Author: Garth Whelan

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