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ADS1248 RTD 2 Channels

jae kwon
Prodigy 45 points
Other Parts Discussed in Thread: ADS1248, ADS1247

Hello.

I'm considering RTD input circuit with ADS1248.

Its datasheet shows an example of three-wrie RTD mesurment circuit(1Ch), but I need 2 Channels input of RTD.

My configuration is same as below.

First RTD is connected to AIN0, AIN1 and REFP0, REFN0 (just like the example of the datasheet)  and then, second RTD is connected to AIN2, AIN3 and REFP1, REFN1.

When converting first one, IDAC1 and IDAC2 will be connected to AIN0 and AIN1.

When converting second one, IDAC1 and IDAC2 will be connected to AIN2 and AIN3.

Is this correct configuration? If there are other good solutions for 2ch RTD with ADS1248, please let me know.

Thank you.

 

  • 0
    TI__Guru* 97755 points

    Jae Kwon,

    The way you describe the connections are correct and is similar to Figure 65 in the datasheet. You could use AIN0-AIN1 for the first RTD and AIN2-AIN3 for the second. The IDACs would then be routed to each channel for the measurement.

    However, you could simplify your system by using the same reference for both channels and use a single resistor. In this case, both RTDs have their third wire connected to the REFP0. Since only one RTD is measured at a time, the other does not have current flowing through it and does not contribute to the other's measurement.

    Also by going to a single reference, you could use the ADS1247 for the measurement instead of the ADS1248. If you do have an interest in putting this together, I would certainly recommend getting the ADS1247 or ADS1248 EVMs to evaluate the performance. A user guide is also available for these EVMs if you want to read further about the use of these devices.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 45 points

    Thank you. It makes perfect sense.

    As another question, Figure 65 in the datasheet says the below.

    "The resistance of the PT-100 changes from 100Ω at 0°C to 119.6Ω at +50°C. The compensating resistor (RCOMP) has been chosen to be equal to the resistance of the PT-100 sensor at +25°C (approximately 110Ω)."

    I cannot realize why the compensating resistor is necessary. I think it is not necessary. Why do I have to make the rsistances equal? Is there any benefit if those are equal or similar? Is it necessary for the noise performance or IDAC1/IDAC2 current matching?

    Thank you

    Jae Kwon

  • 0 in reply to jae kwon
    TI__Guru* 97755 points

    You are correct, the compensating resistor is not necessary.

    It can be used to cancel out the voltage of the RTD so that at a known temperature, the measured voltage is 0V.

    The one benefit that this has is that the ADC is able to measure a smaller signal. In this case, if you decide to use the PGA, you can use a much larger PGA gain.

    Joseph Wu