Re: [Openeeg-list] What kind of shield for cable?

[Andreas R. <and...@st...>]

Hi John,

> But the artikel3 also mentions that the signal wire in
> each of the inputs can pick up mains, too, and that
> this is trickier to get rid of.  For one, they claim
> that the DRL circuit can be unstable (how?) if the
> electrode wires have grounded shields.  

That is because capacitive loading (as in shielding) increases the phase shift in the circuit. I'm a bit hazy on the details of control-theory (Jörg's the wiz), but basically you can cause the circuit to oscillate if the phase shift is greater than 180 degrees at a frequency where the gain is 1.0 or greater. That's actually how an oscillator works: you have an amplifier fed to its own output, with a 180 degree phase-shifter network in between. Think logic inverter. It is an amplifier and phase shift network in one. Connect its input to its output and it oscillates.

> So, finally, my question:  What kind of shielded cable
> should I be buying?  Are there ready-made shielded
> electrode cables?

I am a newbie on EEG, so I can't recommend any brands of ready-made cables to buy or not to buy. If you are going to make your own electrodes microphone cable might work, as it is made to similar requirements. I am using home made electrodes with cheap (< $1/meter) shielded microphone cable. The first half is a 2-lead cable, then it branches into two 1-lead cables, one for each electrode, but that is because I use  2.5mm stereo plugs (same as they use in portable CDs etc) and have one per electrode pair (except ground).

I have had decent results with these in the few tests I have made so far, but I did get a lot of 50Hz noise at first. This was mostly because of high and unbalanced impedances in the scalp. Preparing the scalp better gave better results.

I was not using the DRL here, and from the points you have brought up, we may need driven shields for it. However, Jörg has tested the DRL (without guarding I think) and it worked as it should then, so hopefully we won't need that bit. Fortunately adding a shield driver is simple, if push comes to shove. (However, the analog board design is a bit cramped currently so redrawing the design will be tricky.)

Regards,

Andreas