FPGA Central

Hi,
In the case of an anti-aliasing filter used prior to an ADC, the filte
order is derived from the cut-off frequency being twice the maximu
frequency of the signal, stop-band attenuation and pass-band ripple define
by the ADC resolution.
In the case of DAC, where LPF is basically used to smooth out th
quantisation steps, how do we decide the filter order. It does see
intuitive that the better the resolution of DAC the smaller the filte
order will be. But how do we quantify or derive the filter parameters lik
stop-band attenuation, pass-band ripple and fc.
Thanks.
Krishna

STUPIDENT

krishna_1105 wrote:

> Hi,
> In the case of an anti-aliasing filter used prior to an ADC, the filter
> order is derived from the cut-off frequency being twice the maximum
> frequency of the signal, stop-band attenuation and pass-band ripple defined
> by the ADC resolution.
> In the case of DAC, where LPF is basically used to smooth out the
> quantisation steps, how do we decide the filter order. It does seem
> intuitive that the better the resolution of DAC the smaller the filter
> order will be. But how do we quantify or derive the filter parameters like
> stop-band attenuation, pass-band ripple and fc.
> Thanks.
> Krishna
>
>

On Apr 21, 10:57*am, "krishna_1105" <mv_mail_box-fi...@yahoo.co.in>
wrote:
> Hi,
> In the case of an anti-aliasing filter used prior to an ADC, the filter
> order is derived from the cut-off frequency being twice the maximum
> frequency of the signal, stop-band attenuation and pass-band ripple defined
> by the ADC resolution.
> In the case of DAC, where LPF is basically used to smooth out the
> quantisation steps, how do we decide the filter order. It does seem
> intuitive that the better the resolution of DAC the smaller the filter
> order will be. But how do we quantify or derive the filter parameters like
> stop-band attenuation, pass-band ripple and fc.
> Thanks.
> Krishna

You start with your requirements that drive your stop band
attenuation. Think about what the stop band attenuation will do for
your output. It eliminates the higher order artifacts. So you need
to start with a requirement for the acceptable level for these
artifacts and work backward to your filter order.

Rick

krishna_1105 wrote:
> Hi,
> In the case of an anti-aliasing filter used prior to an ADC, the filter
> order is derived from the cut-off frequency being twice the maximum
> frequency of the signal, stop-band attenuation and pass-band ripple defined
> by the ADC resolution.

Says who? The filter characteristics (and hence it's order) should be
derived from an understanding of the problem. It sounds like you're
applying a cookbook solution that is not only narrow and simplistic, but
very possibly wrong in many cases.

http://www.wescottdesign.com/article.../sampling.html

> In the case of DAC, where LPF is basically used to smooth out the
> quantization steps, how do we decide the filter order. It does seem
> intuitive that the better the resolution of DAC the smaller the filter
> order will be. But how do we quantify or derive the filter parameters like
> stop-band attenuation, pass-band ripple and fc.

You are confusing DAC resolution with sampling rate. Absent of
interpolation techniques (like these:
http://www.wescottdesign.com/articles/sigmadelta.html) the resolution of
the DAC doesn't have much bearing on the required filter.

Given a sampling rate, resolution, and a noise specification, you
quantify and derive your filter parameters by understanding their effect
on the noise, and comparing your estimated noise with the allowable
noise for your system.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html