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How can I converting PT100 resistance on ADS1256

Jeong Jinhyuk
Prodigy 10 points
Other Parts Discussed in Thread: ADS1256, ADS1220

Hello.

I'm Jinhyuk Jeong in Korea.

I am testing the ADS1256.
I have acquired the ADC data.
And I converted it to an int.
But I do not know the current value flowing in the RTD.
So I do not know the resistance value of the RTD.
My conditions are as follows.

1. AVDD = + 5V
2. DVDD = + 3.3V
3. Vref = + 2.5V
4. PGA = 16
5. DRATE = 100
6. Not using Buffer
7. Use a 3-Wire PT100 RTD.
8. I want to measure the temperature range from 50 to 250.

9. Use Stm32 on IAR compiler


Take the data from my terminal as an example.

----------------------------------------------
AIN0 - AIN1 AD Value: 1000ce7a
Stabilizing Value: ce7a
DEC value = 52858
Volt value = 0.001969
---------------------------------------------

I obtained the Volt value. However, I can not know the current value.
Therefore, the resistance value can not be calculated, and therefore the temperature can not be calculated.

Can you help me?

Thank you.

  • 0
    TI__Mastermind 44680 points

    Hi Jinhyuk ,

    Welcome to the TI E2E Forums! I'll be glad to help!

    There are a few different ways of measuring an RTD; however, all of them rely on knowing the value of the current through the RTD, either directly or indirectly.

    • In the direct case, usually you will design the current source (or it will be integrated into the ADC, for an an example check out the ADS1220) and so you know the value of this current.
       
    • In the indirect case, you might have a "ratiometric" circuit, where the current through the RTD also flows through a reference resistor. The reference resistor creates a reference voltage for the ADC and the ADC's measurement no longer depends on the current amplitude, but on the value of the reference resistor! Then, the ADC measurement will be the ratio of the RTD resistance to the reference resistor - so now you need to know the value of this reference resistor (instead of the current amplitude).

    Here is a good resource that might help. In addition to the options I mentioned, it also shows how you measure an RTD in a wheatstone bridge configuration:

    Best Regards,
    Chris