Electronics > Metrology

LTZ1000 digitally controlled oven temeprature

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miro123:
Dear Voltnuts.
I’m new in this forum and initialy posted message in wrong section –
https://www.eevblog.com/forum/testgear/ltz1000-digitally-controlled-oven-temeprature/

I have seen many LTZ1000 voltage references boards.
My three questions,
 1. why nobody do digital control of LTZ1000 oven temperature?
 2. How big is the part of LTZ1000 1/f noise caused by bad temperature control? Does is worth  to make it beter?
 2. Is it an crazy idea to do digital control with such vintage component.? see the notes below
  - modern SD ADC  can sence LTZ Ube with almost unlimited resolution, accuracy and low noise
   - other adc channels can be used to sence data for my model - e.g. several PT100 anemometers etc.
  - Output DAC does not need to be ultra linear. - combined .dual 16..18bit DACs must be enough
  - wide BW noise from digital parts can easily be mitigated
  - DAC ADC Vref can be tricky - for first prototype i can use one temperature controlled KX board.
  - later on with two voltage references I can make even beter digital control.
  - Digital control is area of my interest - once I find the open loop model of my voltage reference board I can buld control loop - I'm prepared that model wil be multidimansional and time variant.

Critics, suggestion or advices are welcome.
I have zero experience with LTZ1000
Best Regards,
Miro

guenthert:
    The reason why it hasn't been done (afaik) is likely that the reference design uses analog control.  Digital control wasn't the obvious choice when the LTZ was developed (and that's the aspect in which its 'vintage' could be relevant).

    As others have already pointed out, the reference design performs quite well, so it's not quite obvious how a digital control can improve on this, particularly due to small distance / low delay between heater and sensor.  The Zener with the moderate TC (50ppm/K, iirc) is clearly the most critical piece to be temperature controlled, but others, especially the current setting and voltage dividing resistors would benefit too (or allow for cheaper components).  I'd rather think a controlled oven for the whole assembly would be the lower hanging fruit (where digital control would be the obvious choice due to the much longer control loop).

TiN:
1. Nobody "publicly". It has been done before by industry.
2. 1/f noise mostly electrical/physical issue, not thermal. Difference would be rather marginal.
3a. SD ADC with unlimited accuracy? :) Do tell, I'd like to have one.
3b. Your ADC in best case as good as your reference is.
3c/d. It is not easy nor cheap to mitigate wide bandwidth noise.

Most reasons why hobby players don't bother with digital control/compensation for LTZ1000-based reference is elegance of analog ways. Adding digital complexity over something like LTZ also introduce all the digital issues, noise, charge coupling, PCB layout impacts, additional filtering, more points to generate parasitic thermal EMFs (extra heat from DAC/ADCs), bigger power consumption (bye bye battery operation). And then spending hundreds hours troubleshooting and revising design to overcome heroically all these challenges, and for what end result? :)

That being said, don't listen too much, and go ahead on trying it. I'm sure it will be fun to learn and something useful can be discovered in the end result  :-DMM

dietert1:
Recently there was a thread on a smart Mosfet driver (TI DRV8320). It has an SPI interface and lets you configure dead time, gate drive currents, current limit etc etc. At TI they seem to be convinced this will be the future. In the past we inserted a resistor or a diode to get it done.
In the case of a voltage reference it would make sense if you had some algorithms or an AI setup to automate "optimization" of a reference. You would need to insert knowledge from eevblog metrology into that AI system. And i guess one would need to make a unit with multiple references, because somehow the AI would need to decide which one or which configuration works better. As far as i understand optimization of voltage references is something extremely rare, due to the long observation periods. A plausible improvement may turn out to be an error only after some years.

Regards, Dieter

Kleinstein:
The problem with the temperature regulation is not with the control loop, but in measuring the difference from the actual temperature to the set point.
The problem only shifts from the divider to the ADC and with modern SD ADCs the divider to divider down the reference from some 7 V  to some 2.5-4 V for the ADC. The demand for the ADC would be quite high. The digitial control would not save much, as the actual control is not the weak point. The standard circuit even gets away with the aquare law heater response and thus power dependent loop gain.

As the LTZ1000 ref. is often used in combination with some high resolution DMM / ADC , it may be still feasible to occasionally read the voltage of the set point divider. If drift is detected there one could still do a numerical correction of the results.  The analog solution is well good enough for the shorter ( < weaks) time frame - the weak point is long term drift of the resistors, that may be limiting. At least the long term drift is not known upfront and hard to test.

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