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RTD Linearization using INA826

Raja Rao Malkari
Prodigy 70 points
Other Parts Discussed in Thread: INA826

Hi,

Based on the application note “Analog Linearization of RTD ”  and the source of http://www.ti.com/lit/zip/SLYT442, I have designed the circuit to measure the temperature(0 to 150­C) using Instrumentation amplifier INA826. By using this circuit, I can able to measure up to 50oC only without any error. But I want to measure up to 150oC. I have implemented circuit which is shown in the file RTD_Linearization_v7.xls as it is. please suggest me where I am going wrong?

thanks and regards

M. Raja Rao

 

 

  • 0
    Guru 21975 points

    Raja,

    The Excel design spreadsheet allows various user-selected values that control the design parameters. Please attach the Excel file containing all the entry values and calculated values that you have used in your design.

    What power supply voltages did you use for the INA826?

    Regards,  Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 70 points

    Hi,

    Here I am attaching the excel sheet of Analog Linearization of RTD. Please find the attachment.

    For 150oC, I have used the following values:

    Power Supply for INA826: I have tested both +5V and +12V

    Vref: +5V

    R1 = 4990 Ohms (>4.9K & <5K)

    R6 = 4990 Ohms (>4.9K & <5K)

    R7 = 99.30 Ohms (>99 & < 100)

    R5 = 143224.812 Ohms(143K)

    R8 = 598.11 Ohms (600 Ohms )

    Gain = 83.593

     

    In RTD place I have place the variable resistor. I have monitor the output voltage by changing the resistance.

    Up to 120 Ohms (i.e. 52oC), it gives correct o/p voltage. But after that it is not giving expected o/p.

    For resistance 157.33 Ohms (150oC) it is suppose to give the o/p of 5 volts but it is giving only 2.1V

    5265.slyt442.zip

    Regards

    M. Raja Rao

  • 0 in reply to Raja Rao Malkari
    Guru 21975 points

    Raja,

    It appears that you may be trying to use the INA826 without a negative power supply. While the INA826 can be operated with a single power supply, it requires special care to assure that internal nodes don't incur voltage swing limitations. Look at the blue trace in figure 12 in the INA826 data sheet. The lower portion of the triangular line shows that the output voltage will be very limited with low common-mode voltage, as is the case with this circuit. This limitation is due to the voltage that must be developed on the output of A1.

    If you try using a -5V supply on the V- terminal I believe you will achieve a more proper output voltage at high temperature.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 70 points

    Dear Bruce,

    Thanks for your suggestion. i have applied -ve supply. now it is working.

    Regards

    Raja Rao