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Mechatronics Zone
An Anti-Alias Filter Review
9/12/2012

This plot shows attenuation for an 8-pole Butterworth filter with a 2,000Hz cutoff frequency.
This plot shows attenuation for an 8-pole Butterworth filter with a 2,000Hz cutoff frequency.

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Rob Spiegel
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Blogger
Why the filter
Rob Spiegel   9/12/2012 1:33:39 PM
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Jon, how much of a problem do you run into if you don't use a filter?

Jon Titus
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Re: Why the filter?
Jon Titus   9/12/2012 1:55:02 PM
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Hi, Rob.  You can get some unusual results because signals at higher frequencies alias, or fold, into the bandwidth over which you want to make measurements. The alising arises from the sampling of signals at discrete time intervals.  Upcoming columns get into more details and explain how to use aliasing to "shift" signals to lower "frequencies" on purpose.  Stay tuned. --Jon

mrdon
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Re: Why the filter?
mrdon   9/12/2012 11:01:22 PM
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Hi Jon,  As always, great technical article. I'll definitely be waiting for the additional articles on Anti-Alias Filters so I can share this information with my Integrated Circuits class at ITT Tech Institute. Keep the tech articles coming!

alexp
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Iron
Re: Why the filter
alexp   9/13/2012 10:57:16 AM
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Hello Rob and All,

IF there are signals with frequency content above the  Nyquist frequency ( half the sample rate ), AND analog anti-aliasing filtering is not used prior to Analog to Digital Conversion, the signals will appear as signals between DC and the Nyquist frequency ...  AND once digitized, the aliased signals cannot be separated nor recognized as being invalid - can be a Major issue.

 From my experience anti-alias filtering is usually a compromise and typically you set the sample rate higher than the desired max needed frequency to allow some  roll-off without significantly compromising the data of interest.

 Modern ADC systems typically sample at their max rate and use  anti-alias filters set  appropriately high... the data is then Digitally low pass filtered and Decimated to create lower effective sample rates with no analog anti-aliasing filter artifacts...

 Alex

Rob Spiegel
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Re: Why the filter?
Rob Spiegel   9/17/2012 10:06:48 AM
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Thanks Jon. So I take it the quality of the data is compromised without the filter. I look forward to seeing the upcoming columns.

Jon Titus
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Re: Why the filter?
Jon Titus   9/17/2012 12:08:25 PM
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Correct, Rob.  Information at higher frequencies (or outside the bandwidth of interest) can interfere with sampled data if not removed with a filter beforehand.

Rob Spiegel
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Re: Why the filter?
Rob Spiegel   9/17/2012 2:05:45 PM
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I take it the filter has to be fairly sophisticated, Jon, in order to avoid filtering out the data you're actually seeking.

Jon Titus
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Re: Why the filter?
Jon Titus   9/17/2012 3:31:47 PM
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Correct, Rob.  You want a steep filter roll off and a response that doesn't attenuate the signals you want.  Stay tuned...

Rob Spiegel
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Blogger
Re: Why the filter?
Rob Spiegel   9/17/2012 4:01:45 PM
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That makes sense, Jon. I look forward to seeing your upcoming columns on this subject.

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