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Order of LPF at DAC output

Started by krishna_1105 April 21, 2009
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


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&#2013266080;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/articles/Sampling/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