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I think the waveform generator would be easier to make accurate and precise over a wide temp range than the ADC would be.
sawtooth waves are easy to generate, and if you feed it through a high gain op amp with lots of negative feedback, the gain will be very precise and stable over temp.
The problem I have with the random noise idea is the random part - random noise is only random over many many samples - over ten, you can get extreems in either direction, so in the short term your signal will be jumping around.
Another way to get more resolution from an ADC is to interpolate the data. If its changing (like an audio waveform) you can oversample the data and interpolate where it would be between the LSBs on the subsamples.
You kinda jumped around in your reply between automotive applications, and what we do on the Bots. I think for the kind of brute force machines we build most of the time for FIRST, 256 degrees of resolution is more than enough anyway :^)
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