CIC filter question (a DC offset at the CIC filter output?)

Dear friends,


I am implemeting a CIC filter for a Delta-Sigma ADC, my question is
that (assuming the CIC filter has one pair of integrater and comb).
Since the input is one bit (0 or 1), the output of the intergrator
will always be that the following sample is larger or equal to the
previous sample. And at the comb part, it subtracts the consecutive
two samples and the result must be always >= 0 (or <= 0). So there
will essentially a DC offset at the output of the CIC filter.

For example, I did a sin wave input (converted to one bit already),
then at the output of the CIC, it is a sin wave but biased at some DC
level. How do I remove the DC or I am missing something here?

Thanks.

"Vincent Ma" <vinma55@hotmail.com> wrote in message
news:d4240f26.0412010312.388162e0@posting.google.com...
> Dear friends,
>
>
> I am implemeting a CIC filter for a Delta-Sigma ADC, my question is
> that (assuming the CIC filter has one pair of integrater and comb).
> Since the input is one bit (0 or 1), the output of the intergrator
> will always be that the following sample is larger or equal to the
> previous sample. And at the comb part, it subtracts the consecutive
> two samples and the result must be always >= 0 (or <= 0). So there
> will essentially a DC offset at the output of the CIC filter.
>
> For example, I did a sin wave input (converted to one bit already),
> then at the output of the CIC, it is a sin wave but biased at some DC
> level. How do I remove the DC or I am missing something here?
>
> Thanks.
Maybe I am missing something too Vincent but if you transklate your input
sine wave to 0's and 1's instead of -1's and 1's aren't you putting a DC
offset on your input?
If so I'd certainly expect to see one at the output.

Best of Luck - Mike