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### CIC Filter Length #2

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```I am reading about CIC filters used for an Sigma Delta converter. In that
the author mentions that

- For a 2nd order filter, decimating by N, the length of the impulse
response is 2N+1

- For a 3rd order filter, decimating by N, the length of the impulse
response is 3N.

- For a 4th order filter, decimating by N, the length of the impulse
response is 4N.

The filter has to operate for a min number of cycles at least equal to the
length of the impulse response.

Can somebody explain how he is defining the length of the impulse response
here?
```
 0

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```On Thu, 05 Aug 2010 16:29:46 -0500, "analog_fever"
<usu_vlsi@n_o_s_p_a_m.yahoo.com> wrote:

>I am reading about CIC filters used for an Sigma Delta converter. In that
>the author mentions that
>
>- For a 2nd order filter, decimating by N, the length of the impulse
>response is 2N+1
>
>- For a 3rd order filter, decimating by N, the length of the impulse
>response is 3N.
>
>- For a 4th order filter, decimating by N, the length of the impulse
>response is 4N.
>
>The filter has to operate for a min number of cycles at least equal to the
>length of the impulse response.
>
>Can somebody explain how he is defining the length of the impulse response
>here?

Hello analog_fever,
I think the impulse response length of a
CIC filter is:

Imp. Resp. length = M(N-1) + 1

where M is the order of the filter.

You should be able, without too much trouble, to
check this using some software.

Goos Luck,
[-Rick-]
```
 0

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