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Analog switch to detach EEG electrodes from circuit

Question asked by Kurt on Mar 14, 2013
Latest reply on May 13, 2013 by Gustavo.Castro

I have what I think is a fairly unusual problem. I have a 2 channel EEG with a right leg drive. I am using AD8226 and AD8607 for the design. At some particular point in time, I need to do the following things with this machine:


- transform the EEG electrodes into direct current delivery electrodes by applying a 30V voltage to them

- disconnect the right leg drive amplifier so that (a) it is not damaged by the voltage and (b) it does not sink current that it supposed to go through the skull


For the electrodes, the amplifier is said to resist more than 30V voltage (from the specsheet) so here I only need a way to connect and disconnect the output of a charge pump to the electrodes. I was told to look for a low capacitance N-channel enhanced MOSFET. However I can't find anything that allows me to switch a 30V signal with 3.3V logic outputs (from my MCU).


For the RLD, here it's more complex because the signal goes both ways so I probably need a double (N and P channel) MOSFET couple.


I am a bit lost because again I can't find anything that works with this kind of voltage. Relays are too big and a SMD reed relay that would work in principle costs 6$ a piece and I'd need 5 per unit so it's out of the question.


Please ask away if you feel the above information does not suffice. thanks.