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Design >> Analog Design >> digital filter / analog filter
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Message started by aaron_do on Jun 28^th, 2007, 7:20am

Title: digital filter / analog filter
Post by aaron_do on Jun 28^th, 2007, 7:20am

Hi all,

just wondering, is it usually necessary to have a digital fitler? I was told that in general the on chip analog filters we design are not good enough for most applications. Therefore, people usually design an analog filter just good enough for ADC, and then use a digital filter since a digital filter is able to better approximate a brick wall response...

Is this true?

thanks,
Aaron

Title: Re: digital filter / analog filter
Post by mg777 on Jun 28^th, 2007, 8:27am

Linear filtering is a mild example of signal processing. You can get 'ideal filtering' by (non-linearly) synthesizing the signal to have the desired properties. For example, a UMTS signal spectrum is a nicely chiseled block, but it wasn't obtained from a brickwall filter.

M.G.Rajan

Title: Re: digital filter / analog filter
Post by carlgrace on Jun 28^th, 2007, 9:43am

Aaron,

That isn't true at all. There are plenty of applications for which analog filters are not only "good enough" but essential. Take for example and RF receiver. There is usually an LNA, mixer, analog filter, then ADC. The question is, how do you spec the required dynamic range of the ADC? Assuming there is enough gain before the ADC, the ADC is specified by looking at the ratio of the peak expected blocker to the desired. In some cases the blockers can be huge. Without a good analog filter, the ADC may require enormous dynamic range and take a lot of power. If you put a good analog filter in front of the ADC, you may be able to knock the blockers down my many dB, and relax the requirements of the ADC. This is much more than simple anti-alias filtering. There are also applications where analog filters are the only way to go due to speed requirements, such as in 10G Ethernet and high performance read channels. There are important uses for both analog and digital filters in integrated systems.

Title: Re: digital filter / analog filter
Post by aaron_do on Jun 28^th, 2007, 6:32pm

thanks for the replies.

I understand that in a RF receiver you would need an analog filter, but is a digital filter necessary? Do we always include a digital filter to perform most of the filtering?

thanks,
Aaron

Title: Re: digital filter / analog filter
Post by ACWWong on Jun 29^th, 2007, 2:19am

aaron_do wrote on Jun 28^th, 2007, 6:32pm:

Do we always include a digital filter to perform most of the filtering?

no, not always necessary, but again depends on your system (signal environment, modulation, demodulation technique etc. etc.).
For example doing demodulation in an FSK (like bluetooth) system using a discriminator, the only channel filtering you'll get will be in the analog domain.

Title: Re: digital filter / analog filter
Post by aaron_do on Jun 30^th, 2007, 12:16am

Hi,

in such a case where all the filtering is in the analog domain, would it then be necessary to reject the interferrers below the noise level? The way I chose the fitler order was based on IIP3 requirements. i.e. i'm not concerned with the blocker's power level at the input of the ADC, i'm only concerned with the power of the third order intermodulation. So if i have all my filtering in the analog domain, i would need to reject interferrer levels themselves below the noise power?

thanks for the help,
Aaron

Title: Re: digital filter / analog filter
Post by Berti on Jul 1^st, 2007, 10:28pm

Hello Aaron,

delta-sigma modulators are a popular ADC architecture for RF receivers. Using a delta-sigma ADC
greatly relaxes the anti-aliasing requirements and the subsequent digital decimation filter removes
shaped quantization noise and residual interferers at the same time. You only need to make sure that the blocker's power
level are low enough to be within the dynamic range of the ADC.
In case of a nyquist converter you probably will need to reject interferers below the ADC noise level to
avoid anti-aliasing.

Regards

Title: Re: digital filter / analog filter
Post by aaron_do on Jul 1^st, 2007, 10:57pm

thanks. Just what i needed to know

Aaron