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RTD Calibration with ADS1248

Fei Yan1
Intellectual 575 points
Other Parts Discussed in Thread: ADS1248

Hello, everyone,

I am using the ads1248 chip to measure the 3 RTD100 temperature signals. I use the 3 differential inputs and 4-wire RTD interface.

The following is some important configuration:

Excitation Current: 250ua

PGA:16

SPS:80

Rbias:2K

I connected 3 same RTD100 sensors. Their metal edges are connected together, so the sensed temperatures should be same. But sometime there is a big difference(maybe not an error) between 2 different RTD sensors.This difference can be more than 0.15 degree celsius.

How can I calibrate these RTD reading numbers to get a good consistency?

The following are some reading figures:

(1)

(2)

The red sensor has a biggest output all the time.

sometime the yellow one is the smallest, sometime the green one is.

I want to know that how can i do to make them stick together?

Thank you

Fei

  • 0
    TI__Guru** 113765 points

    Hi Fei,

    Even a Class A or Class AA, can vary in accuracy by the amount you are seeing.  You need to calibrate each sensor where environmentally there is little drift.  This is often done in a calibrated temperature bath.  You should have at least a 2 point calibration for better overall accuracy.  More points can increase the accuracy.  Depending on which RTD is being measured, you would then need to compensate the measurement by the calibration factor determined from your sensor calibration.

    Something else to be aware of is the input common-mode range and the reference voltage.  Using an Rbias of 2k with 250uA may be marginal with some measurements.  Consider the accuracy of the IDAC source and the tolerance of the resistor.  You want to make sure that both the common-mode is in the proper range for PGA of 16 and the reference voltage also meets the minimum voltage requirements.  If the RTD that is connected to Rbias is outside the common-mode range, then some issues may arise with respect to the accuracy of the ADS1248.  This will become more noticeable with the higher temperature measurements.

    Best regards,

    Bob B

  • 0
    TI__Guru* 98995 points
    Fei,


    Bob brings up an excellent point in that the RTD has it's own accuracy for the measurement. For Class A, I think the accuracy you're thinking of would be difficult.

    Another thing that I want to ask is if there are series input resistors on your board. The input currents may be different from channel to channel depending on the input common-mode voltage. The series input resistance would react to different input currents. This might give different input readings from channel to channel. If there are series input resistances, replace them with shorts and see if this helps match the channels.


    Joseph Wu
  • 0 in reply to Bob Benjamin
    Intellectual 575 points
    Hi, Bob,

    Thank you for your answers.

    For this issue "Using an Rbias of 2k with 250uA may be marginal with some measurements",

    If I change the Rbias resistor from 2K to 5K or bigger, So the reference voltage will become bigger, Vcm will become bigger too. So the Vcm will be in the middle of common input range. This can be a solution for this issue? or how to change the PGA or excitation current to fix the problem?

    what means "the reference voltage also meets the minimum voltage requirements"?


    Thank you

    Fei
  • 0 in reply to Fei Yan1
    TI__Guru* 98995 points

    Fei,


    If you have an Rbias of 2k and use 250uA for the excitation current, this means the reference value would be 0.5V. This is the minimum value of the reference, meaning that the noise performance might be better if you had used a larger value for the reference or the excitation current.

    If you change the Rbias resistor, it's true that you might reduce some of the common-mode error by raising the common-mode input voltage. However, it's more likely that your error comes from another source of error. The gain error in the ADC, the error in the reference resistor, or even the error in the RTD are all possible sources of error. As we pointed out in the previous posts, for a class A RTD, the absolute error is ±(0.15+0.002T), which is where you are now.


    Joseph Wu