Modifica di Electromyography (sezione)

====Input Impedance====
The impedance of a circuit in which alternating currents flow (i.e., currents that do not always have the same direction and intensity over time) is the equivalent of resistance for direct current circuits. The difference is that impedance varies with the frequency of the currents, and thus a filtering effect is generally obtained, whereby certain signals at a particular frequency may be recorded with higher or lower intensity depending on the impedance of the circuits (typically electrodes and cables) through which they pass.

Specifically, to avoid unwanted attenuation and distortion, the impedance of the skin and electrode must be as low as possible, while the input impedance of the amplifier must be as high as possible, so that the current drawn from the biological generator that flows through the external circuits is minimized. Modern electronic circuits allow the creation of amplifiers with input impedances reaching up to 10<sup>15</sup> ohms, with input capacitance on the order of a few picofarads. Considering that the voltage from the surface sEMG signal is on the order of 10 mV, with an impedance of 10<sup>15</sup> ohms, the current in the electrodes and amplifier is minuscule, amounting to only a few thousand electrons (!) per second. However, it is not just the absolute value of the input current into the amplifier that matters: the balance of the currents in all the electrode circuits is also highly important. This requires not only careful amplifier design but also precise measurement techniques.