CM out of range with more than 64 electrodes

[sorenwt]

Hello,

For a study in our lab, we've created custom EEG caps with the standard 64-electrode 10-20 system layout plus additional electrodes to oversample a particular region of interest. However, we've found that when adding a third band of electrodes in the C slot on our Biosemi amplifier, the blue light starts blinking, indicating a CM out of range issue. I've tested it using the bucket test and it's not a bad electrode issue - it happens with multiple strips whenever we plug in a third one.

Looking into this, I thought it may be because of the electrodes we're using - we're using the pin type P32-1020-64A and B, which say they can be used in 64 channel head caps. It might also be that we don't have a true C strip - we're using an extra P32-1020-64A or B in the C slot on the AD box, and the same thing seems to happen when we plug our A strip into the B slot.

Can anyone shed some light on this issue, and (if it really is one of the potential issues outlined above) let me know what electrodes we'd need to order to correct this? Would we need to order just a P32-ABC-C strip or the P32-ABC-A and B as well?

Thanks very much!

[Coen]

You can only use the channels that are actually installed in the AD-box.

To check: inspect the offsets with no electrodes connected to the AD-box, installed channels are maximally negative, uninstalled channels are zero.

You will probably find that only channels 1-64 are installed. In order to use more channels, you will have to purchase additional amplifier/ADC modules.

The electrode sets themselves only differ with respect to the labeling (and sometimes extra integrated CMS/DRL for an A set)

Best regards, Coen (BioSemi)

[sorenwt]

Hi Coen,

Thanks a lot for your reply. This does indeed appear to be the issue - only channels 1-64 are installed.

We have multiple AD-boxes, all of them with only 64 channels installed. However, they do all have the daisy chain input. Is it possible to daisy chain two boxes together in order to record more than 64 channels from the same subject? The issue here I imagine would be having multiple reference electrodes - perhaps we could put the corresponding CMS and DRL electrodes from the different boxes very close to one another, maybe even with a gel bridge between them to ensure they pick up the exact same signal. Do you have any suggestions in this respect?

Thanks very much for your help so far!

Soren

[Coen]

We do not offer a daisy chain option for two 64 channel systems on the same subject.

Best regards, Coen (BioSemi)