Measurement
Here's a comparison of the Sirius® filter to some pure analogue filters:
Illustration 186: Filter comparison: Sirius® DualCore vs. analogue filters
In Illustration 186 above, you can see that the Sirius® filter (orange line) is close to perfection and outperforms any of
the pure analogue filters (see 6.1.2 Filter Design on page 130 for details)
6.1.3.1 Analogue Filter
The analogue filter in the Sirius® modules is a 2
nd
order low-pass Butterworth filter which will switch its
filter-frequency automatically depending on the sampling rate:
when the sampling rate is <=2kS/s, the filter frequency is 5kHz
else 100kHz
6.1.3.2 ADC oversampling
Sirius® will automatically adjust the oversampling depending on the sample-rate.
The digital filter cut off frequency is automatically set to half of the sample-rate and the internal sampling frequency is
up to 256 times higher than the output rate:
Digital Filter (vs. Sample Rate) 0.1kS/s…51.2 kS/s 51.2kS/s…102.4kS/s 102.4kS/s…200kS/s
Bandwidth (-3 dB) 0.494 fs 0.49 fs 0.38 fs
Alias-free Bandwidth DC to 0.42fs DC to 0.32fs DC to 0.22fs
Alias Rejection -95dB -92dB -97dB
Delay through ADC 12/fs 9/fs 5/fs
Oversampling 256 128 64
Note: Sirius® will automatically correct the delay caused by the ADC.
Doc-Version: 1.4.2 www.dewesoft.com Page 133/166
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