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

Bob kb8tq kb8tq at n1k.org
Sat Jun 10 17:04:04 EDT 2017


> On Jun 10, 2017, at 4:10 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Sat, Jun 10, 2017 at 12:59 PM, Poul-Henning Kamp <phk at phk.freebsd.dk>
> wrote:
>> --------
>> Delta-Sigma strategies for spreading the noise-spectrum are
>> interesting, but will not save you if the required heater power
>> ends up being a small rational fraction (1/2, 1/3, 1/4 ...) of the
>> full scale.
> How many output bits are required?  Most uPs have quite a few digital
> output pins.  Each pin could drive a heater resister.  Values of the
> resisters organized by power of two.   Again note the title (poor mans...)
> resisters cost almost zero.  Even of driver transistors are needed you'd
> get change back from a dollar bill.

If you believe that you have a “background” TC of 1x10^-8 / C (for a variety of reasons), that
will drive a few things. For right now simply take it as another arbitrary parameter, just like
the +/- 0.1C we started with. 

If you want the controller noise to *not* be the limiting factor at an ADEV floor of 1x10^-12, 
that drives you to a noise floor of < 0.1 mK. You can then work through the various thermal 
gains to come up with a level of DAC bits that you need. You could equally as well decide
on a 1x10^-13 floor and / or might have a 1x10^-9 sensitivity. 


> The reason I ask "how many bits" is because the above is reasonable with 4
> to 6 effective bits but not reasonable with 24
> I say "effective" because we can dither the low order bits to gain maybe 6
> effective bits form 4 real bits (we can filter the switching noise from a
> low frequency dither)
> -- 
> Chris Albertson
> Redondo Beach, California
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