The McGill Physiology Virtual Lab

Biomedical Signals Acquisition

  There are basically two ways of filtering or altering the frequency composition of the signal. After applying the hardware filter to a given signal, the filtered out frequencies from the signal cannot be recovered. A digital filter changes the frequency content of a signal by performing calculations on the data: the filtered frequencies can be recovered once the digital filter is taken out.


Hardware filtering

From the input amplifier, one can choose the hardware filter to alter the incoming signal,  with the following results:

Left: a 5 Hz sine wave is acquired, with no low-pass applied.
Middle: a 2 Hz low-pass is applied to the signal.
Right: a 5Hz low-pass is applied to the sine wave.

Ideal versus non-ideal filters
An ideal low-pass filter completely eliminates all frequencies above the cut-off frequency (=transition frequency at which the filter takes effect) while passing those below, unchanged. How is it done? by reducing the amplitude of frequency signals above the cut-off frequency. An ideal filter provides a very sharp transition between the frequencies that are passed and those that are filtered out.This in practice cannot be achieved.

A non-ideal low-pass filter passes low-frequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the cut-off frequencies.

One can plot the amplitude response of a low pass filter versus the frequencies of signals and obtain the following curve:
  • Settings on the data acquisition system:

The hardware filter (low-pass) was set to 50 Hz during the entire recording.

  • Settings on the waveform generator:
waveform characteristics: sine waves


 10 fold increments from: 10 Hz to 100 Hz


 constant 10 V peak to peak

Click here to view a Flash movie showing the non-ideal filter behaviour

Click here to view a Flash movie and a quiz
Click here to continue with the next topic, Sampling rate and Aliasing