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

Chris Albertson albertson.chris at gmail.com
Wed Jun 7 14:10:17 EDT 2017

One question for the control theory experts.

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?

It seems to me the problem with fewer bits is only quantization noise.
Lets assume 6-bits.  This is 1 part in 64.   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.

My gut-feel is that this is marginal but could work ("work" is defined as
holds temperature within the range) but I'd be happier using 8 bits.  Im
pretty sure I can get 8-bits by over sampling and filtering.

I don't know how to analyze this but I'm guessing with n-bits each each
sample has a 1/2 bit error so my I and D terms in the PID controller will
accumulate lots of 1/2 bit errors.   I thing I want them "a couple orders
of magnitude" smaller then the  allied temperature range.

Of cose one could buy the best ADC on the market.   But this is POOR MAN's
project.   So he asks, "What is the lowers performance/cost part that will
allow the system to meet its specification?

BTW, a related story.    I'm on another couple lists that deal with vacuum
tube audio.  We see the same things there people correctlypointing out how
to make something better but the question is always how much better and at
what cost an does it matter.   So a fun project was proposed.  Set a budget
of $200 to build a tube based stereo Hi Fi amplifier.  Who can do the
best.  Youhade to publish the BOM with prices and suppliers.   Extra points
if you came in under budget.     This eliminated all the suggestions to buy
high end hand made transformers from Sweden.

IT turrets out that you see MUCH more interesting designs when you lower
the budget.  Anyone can make a high performance system even enough money.
They waste half the cost on useless stuff and the product costs double what
it should and is over complex but is works real, really well.   That's
easy.  Harder and more interesting is "Can you make something just as good
at 1/2 the price?"   Answer is usually Yes.  Then you say "what much do you
loose if I set the price to 1/4?   The answer is surprisingly little if you
get smart about sourcing parts.      Turns out about $180 is the minimum
for pretty decent quality HiFi vacuum tube.

An interesting graph would be Oven Specification vs. Price.  What is the
minimum cost for keeping temperature to within 1.0 C, for 0.1C, 0.01 C?
 Can you do 1.0C for under $5?   or 0.1C for under $10.    I bet yes.

I did an exercise a while back to see what is the minimum price and
complexity to build a GPSDO that was good enough only to drive the lab
bench instruments I have.   I implements only 1/2 od Lars W's design and
cut his lines of code by about 90%.  Turns outhe cost is the XO and about
$10.   Compared to my Thunderbolt, performance was not nearly as good but
the ratio of performance over parts cost might be better.

On Tue, Jun 6, 2017 at 2:39 PM, Mark Sims <holrum at hotmail.com> wrote:

> Another thing to watch out for on processor ADCs is their performance near
> the supply rails...  the AVR ADCs are particularly entertaining below
> around 300 mV (with a 5V Vref).
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> and follow the instructions there.


Chris Albertson
Redondo Beach, California

More information about the time-nuts mailing list