Biomedical Engineering Reference
In-Depth Information
FIGURE 5-47
Interpretation of
aliasing effects in the
time domain.
FIGURE 5-48
Interpretation of
aliasing effects in the
frequency domain.
[Adapted from
(Brooker 2008).]
In reality, the finite roll-off of the filter response requires that a guard band be main-
tained between the spectra. This is achieved by selecting an antialiasing filter with a cutoff
frequency that is less than 0.4 times the sample frequency. Using this ratio as a rule
of thumb, a typical third-order low-pass filter will attenuate these unwanted signals by
between 40 and 60 dB (1
/
100 to 1
/
1000) in voltage.
5.5.4.1 Worked Example: Electrocardiogram
Consider the filtered electrocardiogram signal shown in Figure 5-49. At a glance it appears
to be reasonably noise free.
If the signal is sampled at about 143 samples per second and the spectrum is examined,
Figure 5-50 shows that there is a significant frequency component at 50 Hz. This is caused
by the residual mains hum that was not removed by the CMRR of the differential amplifier.
Because most of the signal content of the actual ECG resides at frequencies below 30 Hz,
it seems reasonable to sample this signal at 75 to 85 Hz.
