Author Topic: LTZ1000 digitally controlled oven temeprature  (Read 464 times)

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Offline miro123

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LTZ1000 digitally controlled oven temeprature
« on: June 08, 2021, 09:33:55 pm »
Dear Voltnuts.
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 
 
 

Offline tszaboo

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Re: LTZ1000 digitally controlled oven temeprature
« Reply #1 on: June 08, 2021, 09:42:10 pm »
They measured the tempco of the un-heated LTZ1000. The conclusion was that the heater sets the temperature with 1/1000 K stability. It doesn't need to be accurate, it needs to be stable. And any variation in temperature would show up on your output as noise. So you would put a sawtooth wave on your reference signal.

Also, blasphemy.
Former username: NANDBlog
 

Offline miro123

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Re: LTZ1000 digitally controlled oven temeprature
« Reply #2 on: June 09, 2021, 06:10:07 am »
Thanks for the answer. It is my first post in this forum and I think I make a mistake with posting. I have selected Test gear section while there is metrology section.  Is there any way to change the topic section metrology?

Direct to your answer
   - All design that I have seen are based on feedback control. Yes the dc loop gain is high and 1/1000 K looks resonable number.
   - the problem with feedback control is that there is delay between disturbance and reaction. LT make fantastic job with integration of Heater zener diode and temperature sensor in one chip. Still delays are in order of hundreds of milliseconds. It takes seconds to fully settle.
   - most design try to minimize the PCB thermal mass
   - other try to insulate the chip. This is equivalent of integrator to external disturbances /air flows etc./
   - all this measures allows to achieve decent temperature control with the limed analogue technique.

My idea is if you have good model of your system you can perform 99% feed forward control. The most thermal and EMF issues will be solved


 

Offline Kleinstein

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Re: LTZ1000 digitally controlled oven temeprature
« Reply #3 on: June 09, 2021, 06:49:58 am »
The actual control theory part is not the problem with the LTZ1000 temperature control. The respnst from the heater to the sensor / die is quite fast (some 10-100 ms time constant), while external disturbance is slow (more like minutges) with a suitable case. So control is easy and not very accurate tuning of the loop is needed.
The difficulty is in the sensor / set point part to have an accurate set-point to the 1/100 degree range over a longer time. So the part where the digital solution may have a slight edge is in the temperature measurement. An ADC would replace the difficult divider for the set point. However there are not so many easy to high highly stable ADCs that ran directly of a 7 V reference. Dividing down the 7 V ref. would already negate the slight possible advantage. So the difficult part would be in the ADC part, measuring the actual temperatur. With a diode's  -2mV/K sensitivity this would be stability to 20 ┬ÁV to get 1/100 K temperature stability or some 6 digit accurcy and stability from the ADC. The noise would need to be soemthing like 10 times better, so in the 7 to 8 digit range to get comparable performance to the simple analog control.
 


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