On 1/27/20 10:32 AM, Charles Steinmetz wrote:
Bob wrote:
Digital pots have *lots* of issues. A high quality wire wound pot
likely will be significantly
more stable and lower noise than your typical digital unit. In
addition the 10 or 20 turn wire
wound will have far more “steps” than a digital pot.
Digipots come in two flavors -- resistive ladders, and multiplying DACs
("MDACs"). Each has lots of issues, some in common and some different.
Even with the MDAC variety (which can have as many as 16 bits worth of
steps), I can't imagine ending up with sufficient resolution to give
satisfactory step sizes for time nuts purposes, unless you cascade at
least two of them in a "coarse and fine" arrangement. Look back through
the archives at the many discussions of suitable DACs for homebrew
GPSDOs, for discussions of how many bits of resolution you need and what
the tradeoffs are [e.g., lack of range]. Resistive ladder digipots are
just hopeless, at around 10 bits of resolution maximum. By the time you
had sufficiently small increments to be useful to time nuts, you would
no longer have enough range to compensate for oscillator drift over a
usefully long period.
Also, look at the temperature coefficients on the datasheets. You see
values in the high hundreds to thousands of ppm per degree C. Not ppb,
mind you, ppm. This, by itself, is very likely a fatal flaw when
trimming measured in ppb is at issue.
Why, "all you gotta do" is just put the digipot in an oven that controls
the temperature to 0.001 degrees. :)
yes, this whole "adjusting by billionths" thing is hard.
That's why as much as I can, I'm going to systems that use "knowledge"
not "control"
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