[time-nuts] uC ADC resolution (was: Poor man's oven)

Attila Kinali attila at kinali.ch
Wed Jun 7 16:40:58 EDT 2017

On Wed, 7 Jun 2017 15:25:38 -0400
Charles Steinmetz <csteinmetz at yandex.com> wrote:

> Chris wrote:
> > Assume me goal is to regulate temperer of an aluminum block to within
> > 0.1C, how good must my ADC be?   Is an effective 6-bits good enough?
> >
> >      *   *   *If I scale the input to the ADC such that it os 1.0C from 0 to 63
> > counts then each cunt is 1/64 C which is about 6 times better then my
> > allowed error of 0.6 C.
> The required number of bits depends on the range it represents -- it is 
> all about scale, as your second paragraph above seems to recognize.
> You are right, 6 bits is enough to represent a 1C span to the nearest 
> 1/64C.  But that is not the whole answer.  This representation will be 
> mapped to 64 steps of oven output.  If the oven has very little power, 
> these can be reasonably fine steps -- but recovery from large errors 
> will be painfully slow.  On the other hand, if the oven has normal-ish 
> power, the steps will be much too large to control the oven temperature 
> finely.

Addedum to what Charles wrote:
If you want to build a temperature control for something similar
like an quartz oven, just get one of the modern delta-sigma ADCs.
You'll pay €10 for one, but it's really a hassle free way to
precisely measure temperature. As most of these have a large
number of channels, you can measure multiple sensors as well
at no additional cost (beside the thermistor).

Additionally: if I would set out to build my own OCXO today, I would
go and buy one of those lunch thermos flasks to house everything. Their
isolation is higher than anything you can easily build yourself,
especially at that size. I would place the (inner) oven at the bottom,
probably using a puck design similar to the E1938, place the electronics
on top of it and close the lid using an aluminium plate which forms
the outer oven. 

Such a design allows to have low temperature gradients within the flask
(due to the metal walls). The outer oven allows to optimize the inner
oven for stability without the need to deal with large temperature ranges.
And all together it is still quite cheap.

One drawback, though, is the large size of the flask. But for a hobby
project that does not need to fit into another product, this is fine.

			Attila Kinali

You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

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