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ADS1247: Application Analysis

Megan Charbonneau
Prodigy 40 points
Part Number: ADS1247

Hello,

My company has implemented an ADS1247IPW in a circuit with a PT100 RTD. Unfortunately, we are getting widely varying readings on the RTD at various temperatures. I am not the circuit design engineer, and am going through a history of ADC and RTD accuracy analysis that was left behind. Currently I am using the SBAS426H datasheet and the SLAU520A reference design doc to double check numbers and calculations made during the initial analysis. Our RTD only has to function in a temperature range from 0 - 43.3 degrees Celsius, and I have determined from Figure 17 in SBAS426H that the greatest gain error for our application is -0.015%.

My question is this: when using this percentage in the Callendar-Van Dusen equations, do I figure in the negative sign when calculating (for example, in equation 62 in SLAU520A)?

I will look forward to hearing back.

Thank you, Megan

  • 0
    TI__Guru* 92715 points
    Megan,


    Thanks for your question on the ADS1247. When it comes to the Typical Characteristics curves in the datasheet, remember that they are typical and not maximum. I also wouldn't assume that they have a positive or negative slope. In that respect, it's better to assume that the error is some plus or minus value. To my recollection however, I do think that the ADS1247 has a predominantly negative gain drift when the device is in higher gains.

    If you are having errors in your system, how large are the errors you're seeing? Is it a large offset or gain error, or is it some sort of noise? If you can, could you report back the data values and what you're expecting? It would be good to have even a basic schematic and the device configuration settings for the analysis.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 40 points

    712B Datasheet.pdf

    RTD-806 Datasheet.pdf

    Module Schematic.docx

    Serno7 Temperature Tolerance Measurements.xlsx

    Serno6 Temperature Tolerance Measurements.xlsx

    https://e2e.ti.com:443/Hi Joseph,Thank you for your quick response, I apologize for taking so long to reply.Let me start off by saying that at this point, we only have a sample set of two modules to test with the RTD. We are using a gain of 128 and an offset of 25. When we probe the circuit, we measure AVDD at 5.023 V, DVDD at 3.30 V, and Vref at 2.473 V, which is what we expect to see. When we use a VI to read the module's response, though, AVDD is reported to be around 4.795 V on average, DVDD at 3.283 V, and Vref at 2.472 V.I have attached two spreadsheets showing the measurements we took with each module, however we used an RTD calibrator to simulate the temperatures that we wanted to test at. I have also included the module schematic, and the datasheets for the RTD we will use with the circuit and the RTD calibrator that we used for this test.You will see that serial no. 6 is able to read the temperatures within 0.6 degrees Celsius on average, but serial no. 7 has a delta of 1.24 degrees Celsius on average. We designed our circuit very closely to the circuit called out in SBAS426G, so we believe that the gain and offset are ok, and are thinking it may be some sort of noise.Please let me know what your thoughts are, or if I may provide any more information that would be helpful.

  • 0 in reply to Megan Charbonneau
    TI__Guru* 92715 points

    Megan,

    Give me a little time with this. It'll take me some time to read through this and I'll likely have some follow up questions. I should be able to get back to you tomorrow.

    Joseph Wu

  • 0 in reply to Megan Charbonneau
    TI__Guru* 92715 points
    Megan,


    There are a few different sources of error that I can see. However, I'm not sure about which error that I'm seeing in your measurement. It depends a bit on how you have it set up. I don't think this is a noise error. This looks like a combination of gain error and offset error and there may be more than one error affecting the measurement. Note that your plots for Measurement Error are generally a line with a slope and offset. If the error were noise, then you would have a jagged line for measurement error.

    First, I have a few questions about the configuration setup. Just to make sure, can you read the configuration register and post the contents? I'd like to know the the IDAC magnitude used (I assume this is a 3-wire measurement and the matched IDACs are injected into two RTD leads).

    Also, do you manually write a value into the FSC register? or do you use the default value? I would use the default value. This value is factory trimmed for each gain setting and adjusts the gain error in the measurement. Second, do you write a value into the OFC register? Your last post implied that you use a value of 25d in the OFC each time. Generally, I would run a self offset calibration to start instead of using some default value.

    What temperature range do you expect to measure? The data shows the equivalent for a temperature range of 0°C to 40°C which is a very small range. I think you want to use most of the full range to easily show how much gain error there is.

    As I mentioned earlier, there are a few possible sources of error. First a mismatch between the two IDAC currents may be a source of error. If the IDAC current driving the RTD is 0.5% lower that the second IDAC current driving the lead, then the ADC give a measurment that is 0.25% lower than expected. This mismatch means that the current driving the reference resistor isn't exactly 2x the current driving the RTD itself, causing the gain error. To counter this mismatch, you can swap the IDAC outputs, and take a second measurement. By averaging the first and second measurements, you can remove the mismatch error.

    I generally avoid using hardware compensation (the 110Ω R137 resistor) in the measurement. Any error in this resistance is seen as an offset error. While you do increase the resolution to include positive and negative values of ADC output data (instead of just positive values), the error and drift of the resistor become additional error terms.

    Are there any other parts of the input circuit not shown here? If there are any ESD diodes or TVS diodes connected to the RTD leads, the leakage terms may be a factor. Any leakage term is taken away from the reference becomes a gain error in the measurement.

    Often, I don't use these RTD calibrators for measurements. I'll generally use 0.01% resistors (measured back with a precision multimeter) for tests. In the application example in the newer version of the datasheet (SBAS426H), there's an example of a 3-wire measurement and the data is taken with precision resistors that have very low drift. If you have some resistors available, then use several values representing RTD resistance across the expected temperature range. Because the error is larger, you might be able to get away with 0.1% resistors, you might have issues with absolute error, drift and self heating. Using a calibrator adds it's own error, and I'd rather use a precision resistor that has a value that I've already measured back.

    Another possible source of error is the series resistance used in the input RC filters. In your case, I don't think this is an issue, because the values are 5kΩ for the input, and about 6.4kΩ for the reference. If the resistors were over 10kΩ or if the IDAC current is small (making the RTD and reference voltages small) then it might be a problem. This resistance reacts with the ADC input current to add an error. If you suspect this is a problem, remove them and replace them with lower resistances or a short.

    Since the error you're looking for is on the order of 0.5Ω in the RTD measurement, you might be able to find the error by verifying that the ADC measurement is correct. Measure the RTD voltage and the reference voltage with a precision multimeter. Note that you'll need a precision meter with a high-impedance mode so that multimeter measurements don't affect the ADC measurements. An Agilent 34401A may work, but a 3458A would be better.

    I know there's a lot to this post, but let me know if any of this helps. When reporting back the data, I'd like to see the raw ADC data (hex or dec, it doesn't matter). Record the input voltages, resistance measurements, and ADC output voltages. Also, I mentioned the application example from the newer SBAS426H datasheet. This application example has a lot of good information about measurements, so read that over for extra clarification. It may add detail to what I've described here.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 92715 points
    Megan,


    I haven't heard back from you for a while, and I thought I'd check up on your circuit problem. I know there was a lot to go through in my last response. If you were able to get a solution, great. If you were not able to find a solution, then feel free to add to this thread.

    I'll leave the thread open for a few days so that you can post a response. If you get locked out, you can start another thread.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 40 points

    Hi Joseph,

    I apologize, I have been sharing your replies with my team and trying to gather some of the info you requested. I would like to answer some of the questions you had:

    Unfortunately, as far as the configuration register goes, I can't answer any of your questions yet because I do not have access to it. I can tell you that we are using a 3-wire RTD, the IDAC magnitude is 1.5 mA, and I do believe that we run an offset calibration on startup. As far as the circuit goes, the schematic I sent you shows the whole thing, and there aren’t any diodes connected to the RTD.

    I am planning on going to the lab this afternoon and tomorrow morning to do some more tests using precision resistors, as you had suggested, and will send you my results. The temperature range that we will be measuring is expected be from 0°C to 43.3°C, however the RTD itself has a range of -50°C to 230°C. My next question for you at this point is, when gathering data using the precision resistors, do you suggest that I use values over the entire temperature range that the RTD can read, or simply widen the range of temperature around what we expect to measure?

    Thank you again for your support and for checking in with me! I will look forward to hearing from you again.

    Megan

  • 0 in reply to Megan Charbonneau
    TI__Guru* 92715 points
    Megan,


    Thanks for the update. I'll wait for more configuration information since it may be important to finding the error.

    For the precision resistors, it's probably best to go through the entire usable range just to see the full gain error, using a few intermediate points to check that there aren't any large non-linearity errors. However, it's probably sufficient to just look at the temperature range that you intend to measure. I think this is large enough of a range to get the information you need.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 92715 points
    Megan,


    I thought I'd check in on you again and see if you had any new data or information about the gain error you were seeing. If you feel like you've got a handle on the issue, I'd like to close out the post. If not, I'd like to see some of the data you may have collected using precision resistors.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 40 points

    Hi Joseph,

    Thank you for checking in. My team and I, unfortunately, have been put on hold on investigating this error any further for the time being. I hope to get back in touch with you at some point, but for now, the post should be closed.

    I appreciate your help and patience,

    Megan

  • 0 in reply to Megan Charbonneau
    TI__Guru* 92715 points
    Megan,


    Ok, thanks. Open a new post when you're ready.


    Joseph Wu