Photodiode Front Ends: The Real Story

Photodiode front ends are by no means glamorous. Living between the detector and the signal processing system, they're supposed to turn a photocurrent into a buffered, filtered electronic replica while preserving the signal-to-noise ratio (SNR). Nobody notices them until they stop doing their jobs. Your optical system may be a thing of great beauty, but a badly designed front end can sink those precious photoelectrons deep in Johnson noise. My unscientific sampling suggests that unfortunately a great many front ends are badly designed: the usual mistake is to trade SNR for speed without a fight. This article will describe techniques for building fast front ends without sacrificing SNR.

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