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..

 

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.

 

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 *)

 

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..

 

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.

 

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..

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 *)

 

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..

 

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?

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 *)