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Author Topic: Clipping of signal and EEG distortions  (Read 2611 times)
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Hi, I start apologizing because I will give any of you that want to help me an hard time.

I am using an EEG device and an alternate current stimulator (tACS).
I have a normal EEG electrode that it is attached both to the EEG amplifier, and at the stimulator, to alternate recording and stimulation, without changing cables

The problem is that when I stimulate the current will flow from the stimulator to the channel and then directly to the amplifier. This will cause two problems:

The stimulation will not flow out to the channel and will not pass to the brain, making all the story pointless.

The amplifier handles brain signals to the range of 1 - 100 milliVolts, while the stimulator stimulates up to 1 milliAmpere. So the impulse from the stimulator coming back will break the amplifier.

My question is:
Can I use arduino to put a resistance in a circuit when the signal exceeds a certain threshold? So activating a switch between the electrode and the amplifier when the signal is too high. So when I am stimulating the current flows from the stimulator to the channel to the reference of the stimulator (on the other side of the head), and when I am not stimulating the signal is not distorted by the arduino?

I am sorry, I am pretty ignorant about arduino and even the simplest physic behind what I want to do. But knowing that it would be possible to realize this with arduino will push me and my group to put the necessary effort in it.

Thanks, Valerio
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1. Passing current into the human body is potentially a dangerous thing to do. Passing current to the brain probably even more so.

2. There are strict medical safety and isolation standards to ensure that an EEG, ECG or anything else connected to the human body cannot deliver a potentially dangerous shock, even when electronic components fail. Don't even think about doing this unless you are familiar with these standards, and the person designing the equipment has experience in meeting them.

3. The problem you describe is commonplace in radar, ultrasonic ranging and similar systems, which have to send a powerful pulse through a transducer and then pick up a weak return signal from it. One solution is to use an amplifier that is tolerant of being overloaded with large inputs and can recover from them quickly, together with an electronic switch between the transducer and the amplifier that cuts off most of the signal going to the amplifier when the transducer is being driven.
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For (2) a battery powered device using a wimpy battery like a 9V PP3 would perhaps be the simplest arrangement - no mains, not
much power or voltage available.

But to get 1mA through the body will take more than 9V, so there are still issues to address.

The EEG signal is high impedance so you can break its connection to the cable with a high-value resistor in parallel with a
suitable capacitor to limit the overload pulses somewhat, and even add a clamp on the EEG side of the RC combination.
Given the high impedances involved the clamp would need to be a JFET or MOSFET I think.

However EEG cable may be a coax-within-coax lead...
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