Why do I need electrodes for ECG measurements?
When recording an ECG signal, the raw signal is taken from the body and sent into an ECG amplifier to amplify the signal to a suitable range and remove common mode and other types of noise.
Measuring the raw ECG signal is done by placing pre-gelled electrodes onto both wrists and one reference electrode on the right leg. The wires connected to the electrodes placed on the wrists are connected to the input terminals of an instrumentation amplifier. So essentially, I am measuring and amplifying the voltage difference between my wrists.
But why exactly do I need electrodes to make this measurement? Why can't I just have two isolated wires touch my wrists and connect those to my in-amp? Why does the interface between my skin and the electrode have to be gelled?
1 answer
The electrodes electrically connect to the skin in order to pick up tiny signals produced by the heart muscles.
Electrically you're a bag of saltwater. Your skin (the bag) has a relatively high resistance. The innards (the salt water) has relatively low resistivity.
To get good signal to noise ratio of something making small signals inside the bag, you want to minimize the resistance thru the bag itself. That gives you better coupling to the source of the signals, and lowers the impedance, which attenuates noise pickup. Piercing the skin and connecting directly to the innards would work, but would be uncomfortable and expose patients to additional dangers.
The best compromise is to connect electrodes in such a way as to minimize the resistance thru the skin. That's done two ways: By using a large surface area, and by adding goo that makes good electrical contact and actually lowers the skin resistance.
0 comment threads
0 comment threads