Drifted Resistance

Discussion in 'General Electronics Chat' started by mhean_ee, Aug 29, 2008.

  1. mhean_ee

    Thread Starter Member

    Aug 25, 2008
    10
    0
    HI..
    Just an inquiry what other factors can affect why the resistance drift after subjecting to elevated temp.

    I have an IC (DAC)...The internal resistance, 1Kohm(for buffer) in the die drifted after subjecting to 125'C, even for an hour. The resistor is a Thin Film Silicon-Chromium. I've seen that the property of this resistor is NTC (Negative temp. coefficient). So I expect the reistance to decrease(resistance drifted to 0.984Kohm) when it's under hot temp. But the value should be returned back again to its value when subjected to 25'C.
    BUt what happened, is that the resistance had drifted permanently.
    Because of that the gain i wanted in my circuit was not properly achieved.

    Kindly help me guys...

    Thanks..
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Older resistors were bad about that. I used to fight something like it when test selecting for some old Collins radios, you might try a more modern type to see if it is still the case, it may be.
     
  3. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    When working with references at Analog Devices (20-30 years ago) we saw
    a permanent drift after the devices were run through the sealing oven. I do not recall the temperature but it was quite a bit higher than 125DegC.

    The solution was to run the device through the oven once to produce the
    shift and then laser trim the resistors to adjust the output voltage. There
    was not much drift when the devices went through the oven a second time.

    (* jcl *)
     
  4. mhean_ee

    Thread Starter Member

    Aug 25, 2008
    10
    0
    The said internal resistor affects the Iout gain. I should only have a resistance of 0.990484Kohm to 1.009516 Kohm to achieve the desired Iout gain. So, the resistance may be drifted, but still should be in this range.

    In this case, is resistor trimming the only way to correct this failure?


    Thanks..
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I've found that resistors usually increase their resistance when exposed to heat; either large amounts for short periods, or lesser amounts for very long periods of time.

    Trimming the resistors will not help; you will only increase their resistance. You cannot decrease the resistance of a resistor by removing material to my knowledge.

    You might, however, decrease the resistance by adding external resistance in parallel with the internal resistance.
     
  6. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    If you bring the DAC to 125degC a second time does the resistance change more?
    If the resistance change is permanent after a single exposure to 125degC trimming
    could work (it worked on the Analog Devices references that I worked on).

    What is the operating temperature range of your DAC? Does 125degC exceed either
    the operating temperature or storage temperature?

    (* jcl *)
     
  7. mhean_ee

    Thread Starter Member

    Aug 25, 2008
    10
    0
    I have'nt tried yet to subject it on 125'C for the second time. I will have an experiment again later and feed it back again to you the result. If the case that the resistance drifted again for the second time, is trimming still be recommendable?

    The storage Temp. range is -65'C to 150'C. And the junction temp. is 175'C.

    Thanks..
     
  8. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    If the resistance changes every time you subject the device 125degC you need to stop
    exposing the device to 125degC or find a different DAC.

    You may want to discuss your application with the IC mfg.

    (* jcl *)
     
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