Understanding power efficiency metrics when selecting analog parts

Rick Zarr over at National Semiconductor is starting up a blog on energy efficiency. His first post takes a look at power efficiency metrics in analog components. His point is that designing an energy-efficient product starts with selecting energy-efficient components, but how do you define energy-efficiency? It’s not a question of just selecting the part that uses the least power, but selecting one that uses the least power while performing its function most efficiently.

He suggests that a better way to compare ADCs is to consider how much error the converter contributes to a given power consumption, which gives a better idea of how good the underlying technology is for efficiently converting AC analog signals to bits. 

Zarr gives equations for measuring efficiency in ADCs, data converters, linear regulators, comparators, and more, as well as rules-of-thumb for what are threshold efficiencies. His white paper on National’s web site has a table of product types, efficiency equations and minimum efficiencies. Note that these equations and numbers are given for National’s Powerwise products, but they hold for all of these product types regardless of the manufacturer.