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Help me with designing an antialiasing filter

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H

houly

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Anti aliasing filter

Hello
I have design an antialiasing filter (14 bits ADC), my bandpass is about 300kHz and the sampling frequency is 1MHz. So Ideally, I would want to have a 84dB of attenuation (14 bits ADC quantum) @ Fsampling/2. This give me a very big filter that can't be realized with analog design. I never see a big filter before an ADC so what kind of filter can I need ? I often see a simple RC before the ADC, is it enough ? and how can I decide the cuttof frequency for my application ?

PS : Order of the filter :
84db/(6*(1E6/(2*314E3))) = 8 see figure 3.8 in the application note :

https://www.google.fr/url?sa=t&sour...zt37CA&usg=AFQjCNEq4LXRjGd5AlUXJXcBA51bV3Dp-w
 

Re: Anti aliasing filter

This give me a very big filter that can't be realized with analog design. I never see a big filter before an ADC so what kind of filter can I need ?
Click to expand...
Something like a 7th order elliptic filter can fulfill the specification. These filters can be realized, either active or passive LC. But not easily. They have been used e.g. for digital audio frontends before the days of oversampling converters, that perform most of the alias filtering in the digital domain.
 

Re: Anti aliasing filter

Hello FvM and thanks for your reply.
Yes it may be realized by LC filter or multiple Sallen key stages, but I never see this ! I know that eliptic filter give a fast transition in gain, but I don't know what is the the phase response of it ... and group delay ? is it a pertinent point for an antialiasing filter ?

Is it REALLY necessary to use this approach (on my precedent post) ?

Thanks a lot for your replies that always are relevant.
 

Anti aliasing filter

None of the classical filters (with a steeper characteristic than bessel) has a good phase respsonse.
That's why digital linear phase filters are preferred.
 

Re: Anti aliasing filter

OK, thanks
but the digital filter acts after the digital conversion, so if we have only a digital filter, we can imagine that noise could pass and saturate the ADC or others thinks that digital filters could not correct ... so we must have an analog filter, does a first order could be fine ? and then a 7th order for the digital filter ?
 

Anti aliasing filter

Yes, that's the way oversampling ADCs work. For classical adc, there's no alternative to chebychev or elliptical flters, if strict alias suppressing is required.
 

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