Which filters are noisier: analog or digital? (Part 1)

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.]

In this article I’ll take a slightly more fundamental look at noise issues in analog-filter design, to set the scene for an eventual show-down with digital filters. (Which will win? How is it going to end? You’ll just have to wait for Part 2!)

I’m going to concentrate on low-pass filters, whose noise bandwidths are essentially determined by their filter responses. Also true of band-pass filters, this isn’t the case with high-pass and band-stop filters, whose noise levels are generally determined by the bandwidths of the amplifiers used.

So, where does noise come from in an analog filter? Well, some of it comes from the op amp(s), for sure. Each active-filter topology has its own particular ‘noise gain’, which causes the inherent input voltage noise of the op amp to be frequency-shaped in a way that is related to – but not identical to – the actual shape of the signal transfer function that the filter creates. This is a fundamental insight useful across all of circuit design: a circuit doesn’t always process its own internal noise in the same way that it processes the inputs you apply to it.

Many op amps have not only an input voltage noise source, but a noise current source as well. The consequence is that any finite source impedance attached to an input of the filter’s amplifier, and therefore passing this noise current, creates an additional noise voltage that also contributes to the overall filter noise. Each topology again has its own signature here, also dependent on the apparent impedances of the resistor-capacitor networks that are hung on the amplifier.

Last, but definitely not always least, is the fundamental noise contribution from the passive components.

To read the entire article, click here.

About the author
Kendall Castor-Perry is a Principal Architect at Cypress Semiconductor Corp., doing mixed-signal system analysis and design for the new PSoC platform.  Kendall uses decades of experience in analog engineering, filtering and signal processing to capture signals across many domains, extract the information from them and do something useful with it.