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How to specify the input and stage wordlength of CIC filter?

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Rozita

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cic filter

question:

How should I specify the input wordlenght and stage wordlenght?

my application:
R=50
M=2
N=2
so from this equation : Bmax= Nlog2 (RM) + Bin
if you estimately consider Bin = 8 bit
Bmax = 21.288
what dose it mean ?
 

wordlenghs of CIC filter

the simplest way is: you use 22bit wide registers in all your cic filter block, including integrator , comb filter.
in the output stage, you can truncate the 22bit wide data to what you desired data width, for example, 8bit, or others.
 

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    Rozita

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Re: wordlenghs of CIC filter

Rozita from this equation : Bmax= Nlog2 (RM) + Bin

or Bmax= Nlog2 (RM) + Bin -1

please clarify
 

wordlenghs of CIC filter

I dont know about wordlength as well @.@ how to choose a correct 1?
 

wordlenghs of CIC filter

My generated results is Bmax = Nlog2(RM) + Bin -1, why ?
 

Re: wordlenghs of CIC filter

The process of bitwidth selection is described in details in the paper:
An economical class of digital filters for decimation and interpolation
by: E. Hogenauer
I guess that the result you have is 1 bit less that you expect because you design an interpolation filter. The upsampling stage of an interpolation filter simply adds R-1 zeros (R is the upsampling factor) between each pair of consecutive samples, so it has an inherent gain of 1/R.
 

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