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Design >> Mixed-Signal Design >> question about the AAF in delta-sigma ADC design
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Message started by min on Jul 26^th, 2007, 1:13am

Title: question about the AAF in delta-sigma ADC design
Post by min on Jul 26^th, 2007, 1:13am

Hi all :

As general says : In designing a single-bit high-order delta sigma modulator , mainly use AAF before the loop filter,to prevent the saturation of modulator.

My question is " If I design only a TWO order single bit delta-sigma modulator , shall I need to design the AAF?"

thanks

Title: Re: question about the AAF in delta-sigma ADC desi
Post by hanm on Jul 26^th, 2007, 1:55am

Yes, of cource you need!
The AAF is used to remove outside band interference. Which makes the input to the modulator a band limited signal. As you know sampling means frequency spectrum perodic occur at the integer times of sampling frequency. Alias occur if there isn't a AAF.

hanm

Title: Re: question about the AAF in delta-sigma ADC desi
Post by min on Jul 26^th, 2007, 2:36am

Hi Hanm :

Thanks for your reply. and i have another question " If in designing a high order loop delta-sigma modulator how the AAF can prevent the loop situation? As the AAF only filter the out of baseband signal ,it do nothing on signal band"

Thanks

Title: Re: question about the AAF in delta-sigma ADC desi
Post by vivkr on Aug 7^th, 2007, 12:50am

Hi min,

You should remember that the modulator will react to signals both inband and out-of-band. Since you are interested in inband signals, it makes
sense to prefilter the input to minimize energy outside your band of interest.

The NTF poles lie outside the signal band of interest and if the input to the modulator contains significant power at frequencies close to the NTF poles,
this can excite it and induce oscillations, which are worse if high-Q poles are used in the NTF. Hence, the need for prefiltering.

For more details, please see Chapter 5 by Bob Adams in the book by Norsworthy, Schreier and Temes.

Regards
Vivek