SIRIUS
®
TECHNICAL REFERENCE MANUAL
6.1.6. SIRIUS® High Speed: Anti-Aliasing
Sirius HS has a fast 1 MS/sec SAR type AD converter. That means that the ADC by itself doesn't filter the
incoming signal, but acquires it »as is« at the time of sampling.
This is very useful in some cases where we want to have bandwidth of the signal as high as possible, for
example when capturing transients, non sine wave (rectangular, triangular) signals, when acquiring
voltage and current from frequency inverters and others.
Then the bandwidth must be as high as possible. With Dewesoft HS-HV and HS-LV the bandwidth of
the amplifier is as high as 2 MHz, ACC module has 500 kHz ...
Of course this kind of amplifier strategy also passes through higher harmonics, which in other cases
(when we are interested in real frequency contents of the signal) doesn't work well. In such cases we
need the strong anti-alias filtering of the signals. What makes it even more difficult is that each
application requires different cutoff frequencies.
SIRIUS-HS can use 3 different filters which can be turned on or off:
- The first one is a 5th order Bessel analog hardware filter with cut-off frequency at 100 kHz,
cutting off the high frequency content in the analog world before ADC conversion. When the
filter is switched OFF, amplifiers will also acquire high frequency content.
- Second filter is FIR anti-aliasing filter/decimator with the cutoff frequency dependent from
sampling rate. This 5th order filter cuts all unwanted frequencies above 100 kHz. On top of that
Dewesoft can set internal FPGA to calculate an additional sharp 8th order filter with variable
cutoff frequencies and shapes. In this case we can be sure that we have a brick-wall alias free
data acquisition with just a simple choice in the software.
- Third filter is IIR filter with selectable cutoff frequencies, orders and type.
Filtering components in SIRIUS® HS modules
Hint
In addition to that the FPGA will downsample the data before sending it to software. Therefore
Sirius is sampling data always with or close to the full speed, filter the data and downsample it,
providing alias free acquisition with any sample rate.
SIRIUS
®
V20-1 289 / 336
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