electrocardiogram (ECG)

An electrocardiogram (ECG) is an oscillogram that provides an electrical representation of muscle activity, including that of the heart muscle. The basic principle of ECG measurement is based on the measurement of voltage differences between human extremities. These voltage differences are recorded via several electrodes and compared with a reference. An ECG measurement can be performed with three electrodes. This also led to the name ECG triangle, named after Einthoven.

In addition to these three electrodes, which form the lead, there is a fourth electrode, the reference electrode, which is placed as far away from the heart as possible, for example on the right foot. The potential differences of the individual leads are recorded against the reference, amplified in an instrumentation amplifier and, after appropriate signal processing, displayed on the screen or printed out via a printer.

The displayed ECD signal consists of the actual ECG differential signal, the offset signal of the electrodes and the differential common mode signal of the electrodes. The differential signal is between the electrodes and is the actual measurement signal. It has an amplitude of about +/-5 mV and a frequency range that is between 0.05 Hz and 150 Hz. There is also a differential offset voltage of about +/-500 mV or higher at the electrodes, which must be compensated.

Since interfering signals are picked up via the human body, the electrode signals are passed through appropriate low-pass filters or notch filters to filter out the interfering signals. They are then amplified and digitized in the analog front end.