On 30 Apr, 19:41, Tim Wescott <t...@seemywebsite.com> wrote:
> nanim...@yahoo.com wrote:
> > Hi,
>
> > In OversampledSigma-DeltaA/DConverter, which is in the demo, why
> > did they use three decimators (4*4*4) rather than one decimator of
> > 64? Is there any particular theory behind this for the gradual
> > process of decimation?
>
> Which demo?
>
> Decimating by four, then four, then four again requires storing 11 past
> values rather than 63, and has each stage running successively slower
> which eases the timing requirements considerably.
>
> --
>
> Tim Wescott
> Wescott Design Serviceshttp://www.wescottdesign.com
>
> Posting from Google? Seehttp://cfaj.freeshell.org/google/
>
> Do you need to implement control loops in software?
> "Applied Control Theory for Embedded Systems" gives you just what it says.
> See details athttp://www.wescottdesign.com/actfes/actfes.html
Thank you for the reply,
the demo is in MATLAB/SIMULINK.
Reply by Jim Thomas●May 1, 20072007-05-01
Jerry Avins wrote:
> nanimerc@yahoo.com wrote:
>> Hi,
>>
>> In Oversampled Sigma-Delta A/D Converter, which is in the demo, why
>> did they use three decimators (4*4*4) rather than one decimator of
>> 64? Is there any particular theory behind this for the gradual
>> process of decimation?
>
> Analyze how sharp the filters need to be for each plan. Remember:
> aliases that appear only in the part of the spectrum that will later be
> filtered are allowed.
>
> Jerry
In other words, if the cutoff frequency of all three filters starts at
the same absolute frequency (in Hz), the first stages can be quite a bit
more relaxed that subsequent stages. They only have to knock down the
frequencies before they alias into the final band of interest. This
makes them far less burdensome computationally. The last stage will
need some steep skirts, but it will operate at 1/16th the original
sample rate.
--
Jim Thomas Principal Applications Engineer Bittware, Inc
jthomas@bittware.com http://www.bittware.com (603) 226-0404 x536
The real fun of living wisely is that you get to be smug about it. - Hobbes
Reply by Vladimir Vassilevsky●April 30, 20072007-04-30
nanimerc@yahoo.com wrote:
> In Oversampled Sigma-Delta A/D Converter, which is in the demo, why
> did they use three decimators (4*4*4) rather than one decimator of
> 64? Is there any particular theory behind this for the gradual
> process of decimation?
Cascaded decimator is efficient: it requires at the order of N*log(N)
operations per output sample, whereas the direct decimation takes about
N^2. This is the same divide and conquer principle, as in the FFT.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by Jerry Avins●April 30, 20072007-04-30
nanimerc@yahoo.com wrote:
> Hi,
>
> In Oversampled Sigma-Delta A/D Converter, which is in the demo, why
> did they use three decimators (4*4*4) rather than one decimator of
> 64? Is there any particular theory behind this for the gradual
> process of decimation?
Analyze how sharp the filters need to be for each plan. Remember:
aliases that appear only in the part of the spectrum that will later be
filtered are allowed.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by Tim Wescott●April 30, 20072007-04-30
nanimerc@yahoo.com wrote:
> Hi,
>
> In Oversampled Sigma-Delta A/D Converter, which is in the demo, why
> did they use three decimators (4*4*4) rather than one decimator of
> 64? Is there any particular theory behind this for the gradual
> process of decimation?
>
Which demo?
Decimating by four, then four, then four again requires storing 11 past
values rather than 63, and has each stage running successively slower
which eases the timing requirements considerably.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●April 30, 20072007-04-30
Hi,
In Oversampled Sigma-Delta A/D Converter, which is in the demo, why
did they use three decimators (4*4*4) rather than one decimator of
64? Is there any particular theory behind this for the gradual
process of decimation?