Hi The “limited range” part of it is why the op-amp makes so much sense. If the ADC can “see” +/-10C that’s way more than will ever be useful. You could make a pretty good case for -5/ +1 C as being way more than you will ever use. The only real issue with the +1 is ok argument is when you offset the oven off turn to balance out the temperature coefficient of the oscillator circuit. Yes, one *could* use a bridge circuit to reduce the need for that offset. I believe it has been suggested and tried :)
Bob > On Jun 10, 2017, at 2:33 PM, Magnus Danielson <[email protected]> > wrote: > > Hi, > > I was about to make this very point myself. The resolution of the ADC needs > to be higher than the limit you try to achieve. There is several ways to > reason about it, but one is that the system is a bit slugish you want to have > higher resolution in order to react of changes before they overshot the > limits you want to keep. Another benefit is that you get away from the > bang-bang behavior you get when having too few bits. > > For an oven you can however cheat some by not requiring linearity in the "too > cold" region of temperature. You do want some linearity as you start to come > into the right range in order to slow down the heating in order not to do a > big overshot. > > I have seen a little too much cases where there been too few bits both on ADC > and DAC sides. Some of it you can overcome, but it runs into trouble. Get > good dynamics, it makes the rest of the design easier. > > Cheers, > Magnus > > On 06/07/2017 08:32 PM, Bob kb8tq wrote: >> 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 <[email protected]> >>> 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 <[email protected]> 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 -- [email protected] >>>> 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 -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
