anti aliasing using the 9233

[jeff_scharpf]

I've done some searching through the forums, and there's alot of good information, but I'm still a little confused about the anti aliasing built in to the 9233.

I guess I'm just looking for some explanations or links maybe.

Here's my situation. I'm using the 9233 to capture sound and vibration (3 channels of vibration and one channel of sound) using some PCB equipment.

I set the sample rate to 50KS/s. I acquire the signal, and send the data into 1/3 octave filters with the low and high frequencies set to .4 HZ and 20KHZ.

My engineer has asked me if I might have any aliasing going on (he doesn't suspect anything yet, just making sure he's getting good data).  I was reading the 9233 spec and it discusses something about sampling very high and automatically providing anti aliasing. If this is the case, do I need to do anything? 

What if I didn't have the 20KHZ high frequency limit on the octave filter?

Just when I thought I understood aliasing, I'm finding it hard to understand and explain to my engineer whether or not I'm giving him valid data with the 9233 and S&V toolkit. 

Any pointers, hints, suggestions, links, etc.. are all greatly appreciated.

Jeff

[Abhinav T · Accepted solution]

Hi Jeff,

The first thing that you should start with is a guide to the delta sigma converter: <link no longer exists>

This is the ADC that is used on the 9233. The delta sigma converter has an inherent anti-aliasing filter built into it. If you look at the dynamic characteristics section of the 9233 operating instructions it will show you some of the characteristics of this filter. The filter is a digital filter and the passband and stopband are calculated based on the sampling rate you specify the 9233. If you give it a sampling rate of 50kHz, the -3dB attenuation point of the pass band will be 50k * 0.45 = 22.5kHz. So any components below 22.5kHz will be preserved completely and anything above that will be attenuated. This ensures that you will not get any aliasing.

If you apply a 20kHz filter after that you will still have good data because the cut-off for the anti-aliasing filter is higher at the maximum sampling rate (50kHz).

Please let me know if this clarifies this issue a little better.

[jeff_scharpf]

Abhinav,

Thanks for the response. Yes that answers all of my questions! It is much more clear now and I feel more confident in my measurements.

Jeff