# CIC filter question

Started by January 17, 2006
```I try to understand how CIC decimator works in presence of DC offset of any
kind at its input? Filter of that tipe passes through all low frequencies
including zero, but integrator stages may be overflowed by DC or low
frequencies. How to prevent this in practice?

Alex.

```
```AAA wrote:
> I try to understand how CIC decimator works in presence of DC offset of any
> kind at its input? Filter of that tipe passes through all low frequencies
> including zero, but integrator stages may be overflowed by DC or low
> frequencies. How to prevent this in practice?
>
> Alex.

CIC filters use modulo arithmetic.  The integrators can overflow
without problem, as long as they have enough bits that each increment
is less than the size of the integrator.

Regards,

```
```
AAA wrote:

> I try to understand how CIC decimator works in presence of DC offset of any
> kind at its input? Filter of that tipe passes through all low frequencies
> including zero, but integrator stages may be overflowed by DC or low
> frequencies. How to prevent this in practice?
>

As long as you are using the same width nonsaturating ariphmetics in the
comb and in the integrator sections, the intermediate overflows are
mutually canceled. You don't have to worry about the intermediate
overflows if the expected output result can be represented with the
given number of bits.

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

```
```>
>AAA wrote:
>> I try to understand how CIC decimator works in presence of DC offset of
any
>> kind at its input? Filter of that tipe passes through all low
frequencies
>> including zero, but integrator stages may be overflowed by DC or low
>> frequencies. How to prevent this in practice?
>>
>> Alex.
>
>
>
>CIC filters use modulo arithmetic.  The integrators can overflow
>without problem, as long as they have enough bits that each increment
>is less than the size of the integrator.
>
>Regards,
>
>
>
Hi, John! Thank you for reply!

I think what will happen with the comb output at the very overflow
moment?
Even though each increment may be much less than the size of the
integrator, its output will "rotate" by a full scale or so and will harm
the comb's output. Isn't it?

Would you like to explane me this in some details. I have learned about
this integrator's property but didn't undestand it.

Thank you.
Alex.

```
```Hi Alex,

Rick Lyons had an article in some on-line magazine
somewhere on CICs which is well-worth reading, but
I forgot where it was. Maybe someone can give the
pointer?

--Randy

```
```"Randy Yates" <yates@ieee.org> wrote in message
> Hi Alex,
>
> Rick Lyons had an article in some on-line magazine
> somewhere on CICs which is well-worth reading, but
> I forgot where it was. Maybe someone can give the
> pointer?
>
> --Randy

Here you go
http://www.eetimes.com/showArticle.jhtml?articleID=160400592

The complete thread on comp.dsp which includes some discussion on overflow
in the CIC is here
http://tinyurl.com/c4uzb

Cheers

```
```Randy Yates wrote:

> Rick Lyons had an article in some on-line magazine
> somewhere on CICs which is well-worth reading, but
> I forgot where it was. Maybe someone can give the
> pointer?

I can't find it either, but here's another intro article:

Martin

--
Quidquid latine scriptum sit, altum viditur.
```
```Randy Yates wrote:
> Hi Alex,
>
> Rick Lyons had an article in some on-line magazine
> somewhere on CICs which is well-worth reading, but
> I forgot where it was. Maybe someone can give the
> pointer?

--
Engineering is the art of making what you want from things you can get.
&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
```
```>Randy Yates wrote:
>> Hi Alex,
>>
>> Rick Lyons had an article in some on-line magazine
>> somewhere on CICs which is well-worth reading, but
>> I forgot where it was. Maybe someone can give the
>> pointer?
>
>--
>Engineering is the art of making what you want from things you can get.
>&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
>

Thank you all! I need some time to think about...
Alex.

```
```junkmail@fastertechnology.com wrote:
> AAA wrote:
>
>>I try to understand how CIC decimator works in presence of DC offset of any
>>kind at its input? Filter of that tipe passes through all low frequencies
>>including zero, but integrator stages may be overflowed by DC or low
>>frequencies. How to prevent this in practice?
>>
>>Alex.
>
>
>
>
> CIC filters use modulo arithmetic.  The integrators can overflow
> without problem, as long as they have enough bits that each increment
> is less than the size of the integrator.
>
> Regards,
>
>

This assumes fixed point processing, correct?

Furthermore, I think it assumes twos-complement representation.

I don't think a Hogenaur(sp?) representation of a CIC filter is
appropriate in floating point.

Anyone disagree?

-- Mark Borgerding
```