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LM35: Using the LM35CAZ as a cold junction sensor for a thermocouple

Greg Jorgensen
Prodigy 10 points
Part Number: LM35

Tool/software:

Hello,

We have been testing the LM35 to see if it is suitable for our cold junction sensor on our thermocouple measurement system. With this sensor we have noticed a Drift over a temperature range from 15°C to 55°C.  Our oven has been calibrated recently. here is a graph of LM35 Error. 

The -3.5°C temperature offset is expected because this LM35 sensor is in an electronics box that generating heat so we would expect the lm35 to be at a higher temperature than the oven. but we noticed that there is drift because the Offset changes as the oven temperature changes. 


1. Is this temperature drift normal for the LM35?

2. is this drift Characteristic consistent across the different LM35 packages? SOIC-8? TO-3 CAN? SOIC-8? TO-220-3?

3. How does the drift Vary within batches(Lot) of the same part number

4. How does the drift vary with different batches(Lot) of the same part number

  • 0
    TI__Genius 11865 points

    Greg,

    How accurate is the temperature setpoint in your calibrated oven? Are you comparing against any other calibrated thermometer? I suspect the drift could be that the oven is not always exactly at the desired setpoint, but just slightly deviating from that as temperature changes.

    Thanks

    -Alex Thompson

  • 0 in reply to Alex Thompson
    Prodigy 10 points

    We see ~0.43C variation when the oven is holding temperature but we pick up this variation in the LM35. We ran one test with a calibrated witness thermometer, attaching a screenshot of the data. But the witness thermometer was not next to the oven temperature thermometer or next to the lm35. 
    we will run a more controlled test in the next few days with a more controlled setup.
       

  • 0 in reply to Greg Jorgensen
    TI__Genius 11865 points

    Greg,

    Thanks for running these tests. I also wanted to ask, are you monitoring for any noise on the LM35 supply line? If large voltage swings/spikes/noise is present on the voltage supply line, the voltage output will be affected. This is described in the Line Regulation spec:

    Looking forward to your additional results.

    Thank you

    -Alex Thompson