SIRIUS®
6.1.2.3 Analogue Filter, Oversampling
& Digital Filter
As a final step, we now apply an analogue filter to the input
signal before the AD converter (which uses oversampling).
Illustration 184 contains 4 diagrams – each of them shows the
analogue filter curve as orange line. The 4 diagrams (from top to
bottom) show:
Input Signal: e.g. the analogue signal that we input to
the AD converter
Analogue Filtered: the output of the analogue filter
Sampled Signal: the output of the AD converter
Digital Filtered: the output of the digital filter (the
filter-input is the Sampled Signal)
You can see that the analogue filter does not have a very sharp
damping – but this is not required. The important thing is, that
the attenuation of the analogue low-pass filter increases for
higher frequencies.
Analogue filtered: The troublesome part of the input signal (red
dashed line) is already highly damped before we even sample it!
Also some other parts of the input signal are damped – but those
parts are way higher than the frequency range, that we are
interested in (f
f
): so this is also not a problem.
Sampled Signal: the sampled signal shows the same folding
effect as before, but this time the aliased frequencies have only
very little amplitude (because of the analogue filter).
Digital Filtered: the final digital FIR filter (with a sharp
damping) further attenuates all signals above (f
f
), so that we get
an excellent measurement result over the whole frequency range
(i.e. the effect of the aliasing is negligible).
Illustration 184: Analogue filter, oversampling and
digital filter
24
Illustration 185 below shows an overview of the components in the Sirius® DualCore and HD modules: after the
amplifier there is an Analogue Filter followed by the ADC which uses oversampling and includes a Digital ADC Filter,
so that we can get excellent aliasing-free measurement results:
Illustration 185: Filtering components in Sirius® DualCore and HD modules
6.1.3 Sirius® DualCore & HD: Anti-Aliasing
To get the best possible anti-aliasing results, Dewesoft uses a combination of multiple technologies:analogue filtering,
oversampling and digital filtering.
Page 132/166 www.dewesoft.com Doc-Version: 1.4.2
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