On Sat, Jun 10, 2017 at 11:33 AM, Magnus Danielson < mag...@rubidium.dyndns.org> 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. > OK, following the advice both above and below. Let's try some real-world numbers... Lets say my goal is regulation within 0.1C. After filtering I have 10 "good" bits in my ADC. That is 1024 counts. My set point is S. I scale the ADC so that 0 == (S - 0.5) and 1023 == (S + 0.5) This means that each ADC count is 0.001 degree C and within the 0.1C range there are 100 ADC counts. But what if there are only 8 good bits after filtering The each count is 0.004 degree C and there are 25 counts within the 0.1C range. The uP's ADC is nominally 12 bits. Getting 10 "good" noise free bits might be asking to much but 8-bits is pretty reasonable. > > 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. >> > -- 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.