Which filters are noisier - analog or digital? part 1

August 01, 2011 // By Kendall Castor-Perry
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.

“Give me a lever long enough”, Archimedes is said to have observed, “...and I shall move the world”. He didn’t 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 Archimedes’s experimental setup on paper, it might look just like a ball of rock being lifted on the plank of a see-saw (that’s a teeter-totter, to US readers) but there’s a little matter of scaling. Let’s 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 you’ll exceed the capabilities of any plank material.

What’s the connection with noisy filters? Well, it turns out that things here don’t scale quite the way you might expect, either. In this two-part Filter Wizard, we’ll look at some fundamental noise mechanisms in filters, using SPICE to illustrate the performance limits you can expect. We’ll concentrate on analog filters in this part, with the biggest bombshell being reserved for Part 2’s look at digital filters. Yes, digital filters generate noise too! And sometimes in unexpectedly, unacceptably large amounts. We’ll 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.

By the way, if you’ve got a practical interest in low-noise filters, I recommend that you get Doug Self’s new book “ The Design of Active Crossovers ”. While it is focused on a specific audio application, the material has wide applicability, and it is a great new addition to the canon of real-world filter cookbooks. In this article I’ll take a slightly more fundamental look at noise issues in analog

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