[SOLVED] electrode-skin impedance mismatch

[matin-kh]

Hi,

there is a term in measuring biopotentials,"electrode-skin impedance mismatch". I cannot understand its meaning. Is there anyone who can explain it for me?

Thanks

 

[BradtheRad]

Impedance mismatch is a term commonly used in conveying AC signals. It's a more complicated sort of resistance than plain DC resistance.

When I grip my meter leads I read about 400k Ohms, which could be called my skin DC resistance. It's rather high. For that reason medical technicians apply conductive gel in order to get better conductivity when they attach wires to your skin.

Biopotentials suggests the aim is to detect electrical signals within flesh. The AC frequencies are weak. Whatever sensor is used to detect them, the aim is to improve efficiency in picking up signals.

I suppose mismatch can mean anything that introduces inefficiency by weakening a signal, or by adding noise. It might be 60-cycle hum in sensor wiring. It might be a faulty ground contact. It might be a particular combination of capacitance and/or inductance somewhere which hampers a desired AC frequency.

 

[Aussie Susan]

Following on from what @BradtheRad said, the 'mismatch' means that if you have a circuit that is supposed to work with a (say) 50 Ohm load and you put a (say) 50 Ohm load on it, then all of the power will be transferred to the load. If the load was (say) 100 Ohm then only some of the power will be transferred and the rest will be reflected back and therefore 'lost'. The same is true if the load is (say) 10 Ohms.
Of course I'm using pure resistance here, but impedance (which is the complex cousin and takes frequency into account) works the same way.
Brads example is exactly what is being talked about in your question. If your circuit is designed to work with a source or load that is very different then you lose power - or in the case of a medical sensor you lose signal. The lower the signal the more you need to amplify and the more noise you can amplify as well.
(When I was at school - decades ago - the example my Physics teacher used was to waggle a rope back and forth. The wave travelled along the rope from one bit to the next because the rope had the same 'impedance' all along it. However at the end was an 'impedance mismatch' and so the energy had nowhere to go and was reflected. This was more clearly demonstrated when you tie the end of the rope to a post or wall and give one quick flick - the wave travels to the other end but is then reflected back along the rope.)
Susan

 

[FvM]

Related to electrophysiology applications which are pretty low bandwidth, impedance mismatch is mainly relevant for balancing and common mode noise suppression with differential signals. A few clicks through respective literature clarifies the point.