Which filters are noisier - analog or digital? part 1
August 01, 2011 | Kendall Castor-Perry | 222902269
The Filter Wizard takes a look at some fundamental noise mechanisms in filters, using SPICE to illustrate the performance limits you can expect. This article will concentrate on analog filters.
Page 1 of 7Give me a lever long enough, Archimedes is said to have observed, ...and I shall move the world. He didnt have the benefit of our modern understanding of materials, which suggests that this boast might be hard to fulfill without a generous supply of Unobtanium. If you drew Archimedess experimental setup on paper, it might look just like a ball of rock being lifted on the plank of a see-saw (thats a teeter-totter, to US readers) but theres a little matter of scaling. Lets say you double the size of everything the radius of the rock, and the width and thickness of the plank. The strength of the plank increases by a factor of four (it goes as width*thickness, roughly) but the load on the plank goes up eight-fold. Keep scaling up, and eventually youll exceed the capabilities of any plank material.
Whats the connection with noisy filters? Well, it turns out that things here dont scale quite the way you might expect, either. In this two-part Filter Wizard, well look at some fundamental noise mechanisms in filters, using SPICE to illustrate the performance limits you can expect. Well concentrate on analog filters in this part, with the biggest bombshell being reserved for Part 2s look at digital filters. Yes, digital filters generate noise too! And sometimes in unexpectedly, unacceptably large amounts. Well see in Part 2 how SPICE noise simulation can be used on digital filters, permitting a direct apples-to-apples comparison of analog and digital filtering solutions.
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