SIRIUS Analogue Filters

Jarman Stephens

Posted on 20.09.2019 04:08

Hi,


I have the following product: SIRIUSi ACC8 purchased in around 2013 (I think). I have a question regarding the analogue filters. I have read the latest Technical Reference Manual (Version 1.5.5) section 6.1 "Filtering" which explains the filtering components in the Sirius DualCore modules very well.


My question is regarding section 6.1.3.1 of the Technical Reference Manual. I would like to manually overide the automatic selection of the analogue filter so that I can have the following settings:

'Sampling rate' = 10kS/s

2nd order low-pass Butterworth filter cut off frequency = 5kHz.

Oversampling factor = 256, which means the sampling frequency will max out at 200kS/s.


I am trying to analyse the frequency range of 0-5kHz. I understand that my desired analogue filter will cause some attenuation (3dB at 5kHz). However, this is still far better than using the automatically set 2nd order low-pass Butterworth filter cut off frequency @ 100kHz. Since, even with oversampling at 200kS/s, the 100kHz Butterworth filter won't have attenuated the higher frequencies much by 195kHz and therefore I will get a lot of aliasing in my 0-5kHz range. (see attached for my messy sketch).


I know that I could get an external low-pass filter and run this into the SIRIUSi ACC8 or get a SIRUIS HS - however I'm trying to avoid having to hire additional equipment.


Thanks and look forward to hearing from you.

Regards,

Jarman


Attached files:
Rok Kmetič
support engineer
Posted on 25.09.2019 07:40

Dear Jarman,


The Sirius Dual Core is the optimal solution in terms of aliasing.

The ADC itself is doing oversampling:



When running with 100 kHz, the ADC itself is sampling in the background with 12.8 MHz.

That’s why the 100 kHz 2nd order filter is suitable for aliasing rejection.

 

We use the combing analog and digital aliasing filter technology.


Kind regards.


Rok Kmetič

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