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rtd reading error ads1248

Other Parts Discussed in Thread: ADS1248, ADS1247

hi,

I am using ADS1248 for temperature measurement in 3 wire RTD Mode. Details of the connection are as in attached file. I am getting reading properly for short lead wires. But there is a drop in temperature reading then expected reading when i connect RTD using more than 1Mtr length. Please help me in resolving the same. Also is it possible to use ADS1RTD CONNECTION.docx248 in single ended mode by connecting the other one channel to ground? 

  • Hi Krishna,

    you may want to read through this TI reference design. This shows our recommended way of implementing a ratiometric 3-wire RTD measurement using ADS1247 or ADS1248.

    Single-ended measurements are unfortunately not possible with ADS1248 when using a unipolar power supply.

    Regards,

  • Hi ,

    Thanks for the reply. I will try the reference design. But i would like to know what may be the possible reason for the above problem, since we already developed application board. Please suggest solution if any?
  • In reference design 4 analog inputs channel were used per 3 wire RTD interface, But for my application i am planing to use 3 set of RTD per IC is that possible? Also since i will be switching IdAC for measurement of channels will it not going to create problem in reference voltage generation?
  • Hi Krishna,

    you did unfortunately not provide a lot of details about your issue, therefore I thought it would be the easiest if you would have a look at the above mentioned TI Design. From your schematic I can for example not determine how your drive the RTD and how the lead resistance compensation is implemented. Could you explain that?
    If the lead resistance compensation is not implemented properly then this could potentially lead to the problems you are experiencing. Do you know what the resistance of your RTD leads is?

    The biggest concern that I would have is that you are not implementing a ratiometric measurement. The IDACs of the ADS1248 are not designed for good absolute accuracy. As you can see from the datasheet the absolute IDAC accuracy can vary quite a bit. The absolute accuracy is however of no concern if you implement a ratiometric measurement approach as explained in the TI Design.
    In your implementation the accuracy of the IDAC would directly impact the measurement accuracy.

    I try to draw a potential solution for you using 3 RTDs with one ADS1248.
    We usually recommend to provide some sort of external RC filter at the ADC inputs. Therefore we route the IDACs around the external filter resistors to not add additional measurement errors. If you don't use any filtering on the ADC inputs then you could route the IDACs to them same inputs that are used for the measurement.

    Regards,

  • Hi Krishna,

    the attached diagram shows a possible solution to measure three 3-wire RTDs using a single ADS1248. You need to switch both IDACs to the other channels in order to measure the other RTDs. In theory this should not cause any problems with the reference voltage generation across RREF. You just need to make sure you give the system enough time to settle before starting the measurements on the other RTDs.

    This solution does not use any input filtering. As I mentioned before, we usually recommend an external RC filter on all analog and reference inputs. However if you would place the filters and route the IDACs around the filter resistors then I do not have enough pins on the ADS1248 anymore (two IDAC output channels would be missing).
    You could use an external analog Mux as a workaround.

    ADS1248 - 3x 3-wire RTD.pdf

    Regards,

  • hi sir,

    Thanks for all details. I tested circuit as per reference design and results are perfect. Now i connected one more RTD to remaining 4 channels as in the reference design itself but results are not ok in both RTDs. I doubt there may be problem with settling time after switching. Presently after configuring each set i am giving delay of 32ms before reading the data, is that ok? Also sampling rate is 20sps.
  • Hi Krishna,

    you are welcome. Could you draw a schematic of how you connected the RTDs?
    In case you are not using any RC filters I would agree that the settling time should not be the issue. The 32ms should be more than sufficient then. Or did you place RC filters now as well?

    Regards,
  • Krishna,


    Along with the schematic, other details would help debug your problem. First, the raw ADC data would help show how much you are off on your measurement. Report back the ADC data in hex and the consecutive data points (and also report back what you are expecting for data values). Does this happen in all channels?

    It would also help to know how you are sequencing the channel changes. Do you write the configuration for the IDAC and the channel at the same write sequence? Do you write them separately? You don't need to show the code, just the sequence of events.


    Joseph Wu
  • yes I am using RC filter with values same as mentioned in reference design.
  • Hi Krishna,

    okay, understand.

    Did you take multiple readings on one RTD after you changed the channel to see if the conversion results settle to a certain value or are they stable right from the beginning? This would give you an indication if you have any settling effects.
    Alternatively you could remove the RC filters to check if this changes things.

    As mentioned above, we would need some more details in order to better help you.

    Regards,