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Famed hardware hacker Bunnie Huang has posted an overview of his notes on designing an open source entropy generator. His summary links to the full notes which include schematics, measurement results, as well as other key details.
The final optimized design takes <1cm2 area and draws 520uA at 3.3V when active and 12uA in standby (mostly 1.8V LDO leakage for the output stage, included in the measurement but normally provided by the system), and it passes preliminary functional tests from 2.8-4.4V and 0-80C. The output levels target a 0-1V swing, meant to be sampled using an on-chip ADC from a companion MCU, but one could add a comparator and turn it into a digital-compatible bitstream I suppose. I opted to use an actual diode instead of a NPN B-E junction, because the noise quality is empirically better and anecdotes on the Internet claim the NPN B-E junctions fail over time when operated as noise sources. I'll probably go through another iteration of tweaking before final integration, but afaik this is the smallest, lowest power open-source avalanche noise generator to date (slightly smaller than this one [PDF]).
(Score: 2) by RamiK on Sunday April 21 2019, @04:49AM (1 child)
Without even looking, power efficiency and heat.
compiling...
(Score: 3, Informative) by Rupert Pupnick on Sunday April 21 2019, @12:35PM
Not a power or heat issue at all. The resistor doesn’t dissipate any power other than from the DC input bias current of the gain stage it’s connected to, which is typically in the microamperes. You pick a large resistor to get a large noise voltage, but not so large as to disrupt the biasing of the input stage.
The underlying principle has been known to communications systems designers since the early days of analog circuit design and can be found here: https://en.m.wikipedia.org/wiki/Johnson–Nyquist_noise [wikipedia.org]