• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 457 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1248: ADS1248 RTD measurement

M_C
Prodigy 80 points
Part Number: ADS1248

Hello,

I have a circuit that is using ADS1248 to measure a 3 Wire PT100 in a temperature range between -50°C and +150°C. The main characteristics are: 

Idac=750µA / Rref=1,1k  / Rcomp=120R  -->    Vref=1,65V / PGA=32

Note that there is a hardware compensation resistor of 120R (middle of the range) in order to maximise the gain. With this schematic, I have a very good and accurate result.

Now I would like to modify this schematic in order to measure a 3 Wire PT1000 RTD in the same range of temperature:


I changed the characteristics as follows :

Idac=750µA / Rref=1,1k  / Rcomp=1200R  -->    Vref=1,65V / PGA=4

This time, I read the resistance measured by the ADS1248, with no calibration (to be sure that the problem is not due to the algorithm of conversion to Temperature beacause I tried few other methods before, I will discuss them later if necessary). 

Problem: For the first few values , there is a stable error of around 0.4%, then the error starts to grow exponentially: 0.4%, then 1.4%, then 2.33%...up to around 4.81% for 150°C

What could be the reason? This configuration was supposed to work better for PT1000 but it is not. Also, because the error is not linear, the calibration algorithm does not work.

Do you have an idea about what could be the problem? What am I forgetting?

Thank you for your help.

  • 0
    TI__Guru* 92655 points
    Mariem,


    There's a lot of people out of the office right now, so I thought I'd jump in and help.

    First, I would generally not use a compensating resistor. Ideally, this would be a good idea to make a measurement that is near zero, to use a higher gain to make the measurement. However, using a compensating resistor requires a precision resistor, with very low drift. In practice, I think you'll get better results without the compensating resistor. I would replace this resistor with a 0Ω resistor and make the measurements again.

    Second, I would make sure that the reference resistor is also a precision resistor with very good accuracy and low drift. Because the RTD measurement is based on this resistor value, the accuracy you get is all dependent on this resistor. A 1% error in the reference resistor becomes a -1% error in the RTD measurement.

    As for additional errors, I would reduce the series resistance for the input filtering for the analog inputs and the reference inputs. Drop them from 10kΩ and 20kΩ to about 5kΩ. High impedances may react with the input current to give a voltage error. Additionally high impedances may disrupt the input sampling (the ADC measurement is done with a capacitive sampling of the input voltage).

    Note that there is a guide for RTD measurements in an application note that can be found a the link below. It may have some extra information for you as you're debugging the circuit.

    www.ti.com/lit/an/sbaa275/sbaa275.pdf

    Again, I would remove the compensating resistor and reduce the input filtering resistors. Then use a precision reference resistor and make the measurements again. If you continue to have problems, make several measurements with different resistances (representing your RTD). I'd like to see all the voltages, and ADC output codes.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 80 points

    Hello Joseph,

    Thank you for your prompt response!

    I realized that I did not give enough information about the circuit.

    All the used resistors in the circuit are low drift, 0.1% error. Also, I am using an emulator to simulate the RTD, so the resistance values are accurate.

    I think that you are right about the compensation resistor, maybe the 1.2K that I have placed is not working as it should (I actually placed 1.1K+100R in series as I did not have a 1.2K).

    Also, I have made a previous test where I left all the hardware as it is, only changed PGA = 1. In that case, I had better values, an error of about 0.65°C to 4.68°C (0.25R to 1.8R), which will probably be reduced with the right calibration.

    Here is what I'm going to do:

    1- Re-test the old circuit (PGA = 1) with a correct calibration

    2- Take off the compensation resistor

    3- Reduce the filtering resistors (if both previous tests are not good enough)

    I'll keep you posted with the results (after the holidays).

    Again, thank you for your help.

    Mariem.

  • 0 in reply to M_C
    TI__Guru**** 168445 points
    Hi Mariem,

    Are your issues resolved now?