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9th February 2010, 10:32 #1
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filter for sigma delta ADC
i am designing a sigma delta ADC. I have completed a second order modulator. My frequency range is upto 40MHz oversampling. My required resolution is 12 bits and so i oversample by 64 times in a second order sigma delta modulator
My question is with the operation of filters... I heard that sinc3 filter and cic filters are best in doing the role of digital decimation filter. Which among the two is best for my specifications?
Also i have a general doubt regarding the output...
The sigma delta modulator gives a 1 bit stream output and the number of 1's will increase if there is a positive slope in the analog waveform and lot of 0's if there is a negative slope irrespective of the position(current analog signals voltage value) in the waveform.
Norma ADC's provide bit stream that directly corresponds to the position (current analog signals voltage value) of the analog signal irrespective of the slope.
How does the decimation filter change this slope indicating sigma delta output to that of a signed magnitude of signed complement form?

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10th February 2010, 14:19 #2
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Re: filter for sigma delta ADC
sinc3 vs cic:
guess it is a tradeoff between complexity and performance. More complex > better image removal and vice versa.
slope to magnitude:
all these filters do are just averaging. More complex filters given you average of the averages many times over. Now finding the average is just integration, if you ignore the scaling by number of samples. Thus the ADC gives you the slope (differential), you integrate and get your real value back by integration.
 B
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10th February 2010, 14:43 #3
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Re: filter for sigma delta ADC
sinc and cic are synonymous, an nstage cic filter has sinc raised n frequency characteristic. They are not necessarily the best,
but the most simple decimation filters, because they don't need multipliers. They are are good suited for applications that don't
need a steep cutoff characteristic. Giving resolution and OSR is not actually a specification in my opinion.
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10th February 2010, 16:48 #4
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Re: filter for sigma delta ADC
ok i get that integration part... but will its ouptut be in signed magnitude or signed complement form
thanks for clearing the doubt over cic and sinc filters
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