Division 2.2 Aliasing errors

In order to avoid uncorrectable aliasing errors, the analogue signals shall be appropriately filtered before sampling and digitising. The order of the filters used and their pass band shall be chosen according to both the required flatness in the relevant frequency range and the sampling frequency.

The minimum filter characteristics and sampling rate shall be such that

1. (a)

   within the relevant frequency range of 0 Hz to f_max = 30 Hz the attenuation is less than the resolution of the data acquisition system; and

2. (b)

   at one-half the sampling frequency (i.e. the Nyquist or ‘folding’ frequency) the magnitudes of all frequency components of signal and noise are reduced to less than the system resolution.

For 0,05 % resolution the filter attenuation shall be less than 0,05 % in the frequency range between 0 and 30 Hz, and the attenuation shall be greater than 99,95 % at all frequencies greater than one-half the sampling frequency.