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ADS1246: Recommendations for ADC Selection

Edward Hunckler
Prodigy 220 points
Part Number: ADS1246
Other Parts Discussed in Thread: ADS124S08, , ADS124S06

Hello,

I'm looking for an ADC to fit the following specifications.

  • It has to measure a PT1000 RTD from a range of -50degC to 300 degC with a precision of 0.001 degC across the range.
    • With the way the circuit is drawn below, the full temperature range comes out to be nearly 0.4 to 1.1V across the RTD using a 4.7kOhm resistor and 3.3V supply.  The number of bits required to measure from -50.000 to 300.000 at that precision is log2(350000)=18.4.  This puts us in the range of a 24 bit ADC.
    • With a 24 bit ADC with the 3.3V excitation, the LSB is ~0.196 uV.  
    • The 350,000 bins over the range 0.4 to 1.1V gives a bin size of 2uV.  So, the 24-bit ADC should be suitable.
    • The above calculations ignore the non-linearity of the RTD, but there is some margin available.
  • The RTD used to make this measurement will also be monitored by a some hardware comparators that are comparing against fixed voltages; so I cannot use internal excitation current sources to make the measurement.
  • I'd like to make a ratio metric measurement of the RTDs using the stable 3.3V rail as the excitation current. Then measuring the signal across channel 0-1, I'd calculate the resistance with the knowledge that we have a precise reference resistor setting up the voltage divider.
I'd like to get suggestions for a 24-bit ADC that would be suitable for this application.  I have used the ADS124S08 to make RTD measurements before, but it seems to be more capable than what is required here.  Most of the nice features like internal current sources will not be utilized due to the hardware monitoring circuit.

I have seen the ADS1246 and that appears to be more in line with what might be required by this application.  Are there other 24 bit ADCs that would be suitable for a single RTD measurement like this?

Thanks for any help!
  • 0
    Prodigy 220 points

    This was the reference schematic that I refer to in the question.

  • 0 in reply to Edward Hunckler
    TI__Mastermind 44851 points

    Hi Edward Hunckler,

    I know you said the ADS124S06 was more than you need, but is there any reason not to use this ADC other than it has more channels / features than is required? I assume you already have experience with this device from both a hardware perspective as well as code. If you switch to a new ADC you will need to write new code as well.

    You could use the ADS124S06 and use the REFOUT feature to bias your RTD using voltage excitation. This way, your signal and your reference (the internal reference) are ratiometric, which should yields the best results. Note that the ADS1246 does not offer this feature as it does not have an internal reference.

    Also, the ADS1246 cannot support a VREF that is the same as the supply, see the image below. Note that VREF (max) must be AVDD-AVSS-1 V, or 2.3V in your case. There are some simple ways to get around this, but I just wanted to point out that the circuit as you have drawn will not work.

    -Bryan