[sdiy] Feedback loop tempco of expo controll!

[René Schmitz]

Hi Karl,

karl dalen wrote:
> How many ADC bits are needed to cover the temperature
> shift that a non temp compensated standard expo causes
> over a temp range of +10 to +45 degC ? 
> 
> Would 10 bits do?
> 
> Looking at Bergfors measurements, 300cent drift of a
> uncompensated expo (25-38 degC )would give at hand 
> that 10 bits might be enough for the 10-45 degC im
> looking for!  No?

Well, what you would need to do is to multiply the CV with a factor that 
is proportional to absolute temperature. Might for example be doable 
with a (two-quadrant) multiplying DAC, however you would waste most of 
the bits for the first 298K, and move only little arround this value. 
(sort of the simplest approach, pure brute force) Say 12Bits would allow 
4096 steps, that might encode 0.1K. The actual resolution to choose is 
dependant on how precise your thermo sensor is.

Or one could factor out a socket amount and only vary a small fraction 
that gets multiplied (then you need four quadrants, not sure if such 
things are available, but you also might do the mult in SW, and spit out 
on a DAC, then with higher precision) and summed back in the end. There 
are probably dozens of configurations that are possible. On some a 
smaller number of bits could suffice.

The idea is appealing in the age of the 8pin uCs. :-P

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159