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 <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).
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Chris Albertson
Redondo Beach, California
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