You should be pretty warmed up for “Ask The Experts” by now. Here’s the last of those three conundra. It’s the one that usually provokes the greatest debate.
People over-think this. In very many cases it really does not matter how a filter transfer function is implemented. You should be guided by what is a good fit on the system architecture that’s already winning your heart/mind on other grounds.
The reason is clear: Signal processing should ideally not be a source of signal path deterioration in your signal. An RF front end, for example, should have an SNR dominated by the first stage. What you do to the signal after that — amplify, sample, filter — should not degrade it. If you follow this guideline, then it simply does not matter in which domain the filter math gets carried out.
When you know that there will be some domain-crossing involved, there’s more of a choice to make. If you’re going to start analog and end digital, and some filtering is needed, you could do it in either domain. However, if the primary purpose of the filtering is to elevate the quality of the subsequent conversion to digital, then you just answered your own question — look at doing the filtering in the analog domain — or at least enough filtering that the conversion quality is good enough.
Likewise, if you’re going to start digital and end analog, consider whether the process of converting your signal back to analog is going to generate additional stuff you don’t want. The architecture of the DAC you use plays a role here. Delta-sigma DACs tend to produce much less “hash” around the sample rate than regular sampling DACs. But, then, that’s because the manufacturers of those delsig DACs already built the filtering in, so you don’t have to.
And that highlights another issue. You may not have to do any filtering yourself, because the very helpful manufacturers may have integrated it into one of the devices you plan to use. The most you have to do then is just feed the device some configuration parameters. Sometimes not even that. This is how the majority of audio filtering is done these days: a little farm of transistors grazing away on your signal inside a chip somewhere, with zero effort on your part.
Hopefully your gray matter is now alight with questions and comments about filtering. Let 'em rip! And see you on the 22nd!
Please join us on Wednesday, October 22nd, at 1:00 p.m. ET (10:00 a.m. PT) for a chat session in which we will discuss “Filter Design.”
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