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Sampling frequency and Nyquist theorem - Data acquisition

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Mari,

 

There is no way, theoretically or practically, to reliably identify aliased signals.  Sometimes, when the frequency of the signal which creates the alias is precisely known you can calculate where the alias would appear. Even then it is impossible to separate that component from any portion of the real signal which appears at that frequency.

 

If you think there is a possibility of aliasing the only real fix is an external hardware anti-aliasing filter.

 

Lynn

Message 11 of 13
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I used NI 9201 as my DAQ card to read my piezoelectric sensor (unimorph). my cut off frequency is supposed to be 500 HZ but could not insert the value in the filter (as it already mentioned that it has default only 400 HZ). and it keep appearing in my filter that " the current filter specifications do not meet the Nyquist criterion for the given input signal" . How do i solve this problem and is there any mistake with my procedure?

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Message 12 of 13
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@nurfikhira wrote:

I used NI 9201 as my DAQ card to read my piezoelectric sensor (unimorph). my cut off frequency is supposed to be 500 HZ but could not insert the value in the filter (as it already mentioned that it has default only 400 HZ). and it keep appearing in my filter that " the current filter specifications do not meet the Nyquist criterion for the given input signal" . How do i solve this problem and is there any mistake with my procedure?


  1. You should start a new thread.
  2. I don't use DAQ Assistant and have no idea how it transfers to another computer, but when I open it, it says the sample rate is 500Sa/s.
  3. For that rate Nyquist is 250 Hz.
  4. You cannot set your low-pass to 250 Hz, it needs to be less then Nyquist, so the max would be 249.99... I think you are better off setting it to 200 Hz.

mcduff

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