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LM324: Temperature drift characteristic of LM324 is different from the specification

Kyutae Kim
Intellectual 330 points
Part Number: LM324
Other Parts Discussed in Thread: LMV324, TINA-TI, TLC2274

Hell All.

I built RTD sensing circuit as attached RTD.tsc TINA file and have tested the output voltage drift of the LM324 about room temperature changes to check if the voltages are correct in comparison with TINA simulation result. Unfortunately, temperature characteristic of LM324 in the physical board is totally different from the simulation result with TINA.

I simulated the circuit about room temperature change from -20 degree to 40 degree, and have obtained the drift profile as attached below increasing monotonically. But in the board, the output voltage is decreasing with room temperature increasing. This occurs identically among other same boards.

I've also tried to test it with LMV324 using same board by just replacing the OP amp, and found the output voltage increases correctly as the room temperature increases just like the TINA simulation results.

Could you help me understand why this circumstance occurs or if this situation seems to be normal?

* FYI, I checked the voltage in the board for some temperature points with a multi-meter, so no measuring data exists in the form of the graph.

Thanks in advance.

RTD.TSC

  • 0
    Guru 140200 points
    Hi Kyutae,

    because in a RTD sensoring circuit the measuring signal is very small you should only take precision OPAmps for the signal processing. The LM324, on the other hand, is a very low cost OPAmp not offering the precision you need. If you look into the datasheet of LM324 you will notice that the temperature drifts of input offset voltage, input bias current and input offset current aren't specified at all. So, it's questionable whether the TINA-TI simulation gives proper results.

    Kai
  • 0 in reply to kai klaas69
    Intellectual 330 points

    Hi, Kai.

    Thanks for the reply.

    Then do you think TINA-TI simulation result could be incorrect to the LM324? How about the LMV324? Do you think the LMV324 could also have same problem in terms of the voltage offset drift, and should I take the precision ones instead? As you might know, precision op amps are more expensive than LM324/LMV324, so I would try to escape to use the precision op-amps as I can as possible.

    Additionally, I come up with new question to the TINA-TI from your answer. I had also simulated the TLC2274 with same circuits as attached, however the output voltages increased dramatically in that case despite of its spec. Do you think I should not fully trust the simulation results of the TINA-TI during developments?

    RTD_TLC2274.TSC

  • 0 in reply to Kyutae Kim
    Guru 140200 points
    Hi Kyutae,

    I understand you. But you usually get what you pay for...

    First, think about what kind of precision you need at all. What is your worst case tolerable error? Then, make an error analysis by hand with the specifications of datasheet of OPAmp. This will show you very quickly whether the choosen OPAmp is suited or not.

    TINA-TI is a very nice tool. But it cannot simulate every parameter! So, these simulations are very useful but they don't tell you the "ultimate truth"...

    Kai
  • 0 in reply to kai klaas69
    Intellectual 330 points
    Thanks Kai! I understood.