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OPA244 variability in temperature

Pierre Dabreteau
Prodigy 70 points
Other Parts Discussed in Thread: OPA244, XTR111, OPA237, TINA-TI

Dear Sir ,

I use one OPA244 for generated a 4-20mA output but with this OPA a don't have a stability in temperature

Have you a solution or another OPA???

Thanks

Your faithfully

 

  • 0
    TI__Mastermind 27715 points

    Hello Pierre,

    Have you considered the XTR111? It is a 4-20mA transmitter which generates a current output based on a voltage input, and it also has good stability vs. temperature.

    What is your application? What are you currently using to drive the OPA input? Are you using a 4-20mA receiver or some other load?

    Best regards,

    Ian Williams
    Linear Applications Engineer
    High Performance Linear

  • 0 in reply to Ian Williams
    Prodigy 70 points

    Dear Sir,

    I just search a OPA who was stable in temperature because with the OPA244 I have  0.40mA variation between -20°C and 60°C

    I join the schema of my 4-20mA. 

     

    Thank you for your reply

     Best regards

     

    Pierre

  • 0 in reply to Pierre Dabreteau
    TI__Mastermind 27715 points

    Pierre,

    The OPA237 is very similar to the OPA244, but has better stability with respect to temperature.

    That being said, the temperature coefficients of all the components around the op-amp will also have an impact on your overall performance. Make sure to select resistors and capacitors with as little temperature drift as possible.

    Best regards,

    Ian

  • 0 in reply to Ian Williams
    Prodigy 70 points

    It's the same probleme with the OPA237

  • 0 in reply to Pierre Dabreteau
    Guru 21975 points

    Pierre,

    Ian is traveling so I have looked at your schematic and have the following comments:

    A SPICE simulation of your circuit using TINA-TI shows that the output current is not particularly sensitive to offset voltage changes of the op amp. A 1mV change in offset voltage of the op amp (a larger than expected change) produces only a 30uA change in output current. I do not believe that the op amp is the cause of variation in output current.

    Using the nominal resistor values shown in your schematic, the circuit is sensitive to power supply voltage changes. Unless you are carefully controlling the power supply voltage, you should expect significant variations. To achieve output current independent of the supply voltage, you should make changes to the resistor values. The ratio of R64 to R65 should be equal the ratio of (R62+R63) to R65. Thus, if only one resistor is adjusted, R65 should be 103.503k ohms.

    In addition, the circuit is very sensitive to variation in resistor values. A 1% change in the value of R65 from its optimum value produces nearly 1mA change in output current. Temperature sensitivity of the resistors may be the primary cause of variation. Very precise and stable resistors must be used.

    Another possible source of variation may be related to the output voltage swing capability of the op amp. The output of the op amp may not be able to swing high enough to fully turn off T3 at low currents. I could see this effect in simulation but the accuracy of the macro model and temperature variation of this behavior is uncertain. I removed this potential effect in simulations by placing a 1V source in series with the output of the op amp.

    Yet another possible source of error could be the DAC that is driving the circuit. You have not provided a part number but it appears that its output is ratiometric with the 3V power supply. If so, then this power supply must be very stable.

    You may want to consider a device such as the XTR111 that uses an internal topology that is fundamentally far more stable.

    Regards, Bruce.