User manual



Filters
Filtration is an inseparable part of signal processing. A filter is generally a circuit
designed in such a way as to transmit signals of a certain frequency bands, while suppressing
other frequencies. These circuits may be realized via passive components, such as resistors,
coils and capacitors, or via active components, such as amplifiers.
Fig. 2.12: Connection of the Driven Right Leg circuit.
When processing the ECG signal, a frequency band of 0.05 Hz - 120 Hz is used. A high-pass
filter with limiting frequency of 0.05 Hz removes disturbing elements from the signal, caused
by slow movements of the examined person and slow electrochemical action.
Low-pass filter, set to the position of 120 Hz, defines the upper threshold of the
transmitted frequency band, thus removing the disturbing elements outside the scope of
frequencies of the wanted signal.
The ECG record contains a very significant disturbing contribution from the mains
voltage, oscillating around the value of 50 Hz. To remove it, the so called band-reject filter is
used. This serrated filter, the so called Notch filter, suppresses only one required frequency, or
eventually a very narrow band of frequencies.
Isolation amplifiers - galvanic separation
Galvanic separation of the signal is realized via isolation components (such as
isolation amplifiers, optocouplers). These components ensure the transmission of signal,
analogue or digital, from the input to the output via an isolation barrier. The barrier ensures
that there is no galvanic (ohmic) connection between the input and the output. The isolation
28