What is the working principle behind these kind of sensors? I'm talking about the type used for heart monitors, EEG machines and such like. How are these most delicate signals sampled without flattening them in the process?
I believe that EEG and EKG machines sense electrical signals directly, without specific "sensors".
Someone who does medical can give you a better answer, but the first step in accurately measuring the voltage of a high-impedance source is to use an amplifier with an even higher impedance.
-- www.wescottdesign.com
Instrumentation amplifiers - high in Z, high CMRR and low noise are used. EKG signals are much larger than EEG so requirements aren't as stringent.
-- Grizzly H.
"Heart monitors" generally use large-area electrodes with fairly low impedances. EEG electrodes are smaller, so somewhat higher impedance. As MixedNuts said, you use amplifiers with much higher input impedances than the electrodes.
There are many other kinds of "voltage" electrodes - the glass micropipettes and the metal microelectrodes with really small active surface areas; those used in patch-clamp systems; and some of the biochemical and pH electrodes with really insane impedances - now those are challenging! The "working principle" varies on the type of electrode.
direct connection
instrumentation amplifier
NT
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