I am implementing some custom modifications to Actiview, in order to make it capable to perform some online analysis and triggering. As a part of the developement and testing, I need to connect a small signal generator to some electrodes of the Active Two. My question is, which is the advisable way to make an electrical connection? So far I used to dip the electrodes in conductive gel, wrap them aluminium foil, and connect crocodiles to the foil. Typically I join DRL and CMS as ground, and some other electrode as test input. It works, but the foil gets corroded in a matter of one-two days, eventually loosening the electrical contact. I figure out that this is because dispersed currents and electronegative potentials of the aluminium and of the electrode compound (which?), and I feel better seeing that it is the foil which gets the worse part, but what is really happening? Can anyone recommend a better way?
Thanks, Enrico
Best way to connect a generator to the active electrodes
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Enrico Segre
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Coen
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The best way to connect electrodes to a signal source is to use a bucket with salt water. Use a non-metal bucket (plastic or glass) to prevent pollution of the electrodes with strange metal ions. We have found no adverse effect from using simple tab water and household salt. The concentration is not critical: one spoon per liter is about right.
Insert all electrodes in the bucket (blue LED should turn on). Signal currents can now be induced in the water by "connecting" a signal generator with a pair of conventional (passive) Ag-AgCl electrodes. The amplitude of the signal will vary among the channels, due to the relative location of the active electrodes with respect to the signal generator electrodes.
If you need a defined signal amplitude, you can use two buckets of water. Locate the CMS and DRL in one bucket, and the other electrodes in the other. One signal generator electrode goes to one bucket, and one to the other. The unreferenced display then shows the generator amplitude. With this method, you can for example check the amplifier calibration.
We strongly recommended against making contact to the electrode tips with metal foil or alligator clips. These methods are likely to pollute the electrode tips with (non-silver) metal ions, which might lead to noisy electrodes. Polluted electrodes can sometimes be "repaired" by soaking them in salt water for a long time (12-24 hours), until the all pollution is dissolved in the water.
Best regards, Coen (BioSemi)
Insert all electrodes in the bucket (blue LED should turn on). Signal currents can now be induced in the water by "connecting" a signal generator with a pair of conventional (passive) Ag-AgCl electrodes. The amplitude of the signal will vary among the channels, due to the relative location of the active electrodes with respect to the signal generator electrodes.
If you need a defined signal amplitude, you can use two buckets of water. Locate the CMS and DRL in one bucket, and the other electrodes in the other. One signal generator electrode goes to one bucket, and one to the other. The unreferenced display then shows the generator amplitude. With this method, you can for example check the amplifier calibration.
We strongly recommended against making contact to the electrode tips with metal foil or alligator clips. These methods are likely to pollute the electrode tips with (non-silver) metal ions, which might lead to noisy electrodes. Polluted electrodes can sometimes be "repaired" by soaking them in salt water for a long time (12-24 hours), until the all pollution is dissolved in the water.
Best regards, Coen (BioSemi)
Re: Best way to connect a generator to the active electrodes
If you want to check the gain specifications of the ActiveTwo system, then you can do this with the two-bucket test:
You will need the following:
- signal generator
- 2 passive AgAgCl electrodes that can be connected to the output of the signal generator.
- 2 plastic buckets or 2 glass bowls (never let any metal touch the electrodes)
Put the CMS and DRL in one bucket and all P32 sets (32, 64, 128 or 256) in the other bucket. Use the signal generator and two passive Ag-AgCl electrodes to connect a 100 mVpp 10 Hz signal between the buckets. Then verify that ActiView shows a 100 mVpp on all 64 channels in unreferenced (raw) display. Then select "average of 64 chan" as a reference. ActiView now shows the difference between each channel with respect to the average. The difference should be smaller than 1 mVpp (1% of the 100 mVpp input signal).
You will need the following:
- signal generator
- 2 passive AgAgCl electrodes that can be connected to the output of the signal generator.
- 2 plastic buckets or 2 glass bowls (never let any metal touch the electrodes)
Put the CMS and DRL in one bucket and all P32 sets (32, 64, 128 or 256) in the other bucket. Use the signal generator and two passive Ag-AgCl electrodes to connect a 100 mVpp 10 Hz signal between the buckets. Then verify that ActiView shows a 100 mVpp on all 64 channels in unreferenced (raw) display. Then select "average of 64 chan" as a reference. ActiView now shows the difference between each channel with respect to the average. The difference should be smaller than 1 mVpp (1% of the 100 mVpp input signal).