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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> digital filter / analog filter https://designers-guide.org/forum/YaBB.pl?num=1183040429 Message started by aaron_do on Jun 28th, 2007, 7:20am |
Title: digital filter / analog filter Post by aaron_do on Jun 28th, 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 28th, 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 28th, 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 28th, 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 29th, 2007, 2:19am aaron_do wrote on Jun 28th, 2007, 6:32pm:
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 30th, 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 1st, 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 1st, 2007, 10:57pm thanks. Just what i needed to know Aaron |
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