Hi There is a gotcha with the initial assumption: You want the loop to be *quiet* at a level well below 0.1C. If it is bouncing around that much, the second order (rate defendant) tempco of a normal crystal will become a pretty major issue.
Simple rule of thumb - add at least two bits past whatever the target is. More or less, if you *are* after 0.1C and that comes out to 6 bits, you need eight solid bits to get things to work properly. Bob > On Jun 7, 2017, at 2:10 PM, Chris Albertson <albertson.ch...@gmail.com> wrote: > > 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 <hol...@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@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 > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.