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ADS1248: Interfacing the LMT85 as a cold junction sensor

Tony Merolla
Prodigy 120 points
Part Number: ADS1248
Other Parts Discussed in Thread: LMT85, , LMT70, ADS1220, ADS124S08

An ADS1248 design uses an LMT85 as the cold junction sense for thermocouple measurement.  The ADS1248 is unipolar supplied.  In order to comply with ADS1248 measurement requirements, the signal needs to be 100 mV above AVSS (AVSS + 0.1).  Hence,, based upon this and other reading from this Forum, a single ended measurement is not appropriate for this ADC system.  Is it acceptable, therefore, to basically lift the ground of the LMT85 by say 200 mV or so and make a differential between LMT85's Vout and the lifted ground?  (Basically, what is done when using internal bias to lift the thermocouple leads to mid-supply, but not mid-supply.

If this is a valid method, is using a voltage divider with an op amp buffer to set the 200 mV a good approach?  The PGA would be set to 1.  All measurements should be ratiometric.

As a curve ball, a TI application report, SNAA267, April, 2015 shows an LMT70 interfaced to an ADS1220, Figure 13, page 10.  Reviewing the ADS1220 data actually requires AVSS + 0.2 V.  However, unless there is some other magic I am not seeing in that app note or data sheet, how does that circuit not comply with requirements?  I shall go on the stretch that the LMT85 is measured with respect to AVSS, which is part of single ended measurement and unipolar supply.

Thank you.

  • 0
    TI__Guru* 92715 points

    Tony,

    Just reading through the LMT85 datasheet, you could do as you described, and use a voltage divider. If you did a voltage divider of 1/2, the at the maximum temperature, the output voltage from the LMT85 would be 300mV, so that the negative input for the ADS1248, would be at its lowest 150mV. That would keep the input within the range of the PGA, at gain of 1. I have a diagram below.

    There might be some amount of CMRR, but that wouldn't affect the measurement very much. The other issue that I see is that the output drive of the LMT85 is rather small (only about 50uA). This means that you'd need to have large resistors, and the input current of the ADC may become a factor.

    An alternate way of performing the level shift would be to place a diode below the LMT85. This would shift up the voltage a diode above the ground without having to make a divider and worrying about the load of the resistors..

    If you do decide to go this route, I would get an LMT85 and test it to be sure it works. I've never used the LMT85 (I'm much more familiar with the ADC). It might be worth posting into the Temperature Sensing Forum to see if anyone has done this before.

    As for the LMT70 in SNAA267, you are correct that the ADS1220 does require that the input stay within 200mV of the positive and negative supplies. However, this is only when the PGA is in operation. Unlike the ADS1248, the ADS1220 allows the user to bypass the PGA to make measurements directly with the ADC. With the PGA bypassed, the input range goes from 0.1V below the ground (with a unipolar supply) to AVDD+0.1V. Also, even if the PGA is bypassed, the ADC can still make measurements with gains of 1, 2 and 4 (through input and reference scaling).

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 120 points
    Thank you, Joseph. A diode may be a dandy fix. Easy enough to try. Additionally, appreciate clarification of bypass feature of the ADS1220. That information will rattle about in my head for future reference.

    Tony Merolla
  • 0 in reply to Tony Merolla
    TI__Mastermind 20845 points
    Hi Tony,

    as it seems like the output voltage of the LMT85 is >=300mV over its specified temperature range.
    An easier implementation would then be to measure deferentially between the LMT85 output and the VREFOUT of the ADS1248. Means connect one analog input to the LMT85 output and one analog input to VREFOUT. That way you would meet the input voltage requirements of the PGA. The accuracy of the internal voltage reference might however slightly degrade your temperature measurement accuracy.

    BTW: In the next generation of ADS1248, the ADS124S08, we did implement a PGA bypass option just like in ADS1220 to make single-ended measurement using a unipolar supply possible.

    Regards,
  • 0 in reply to Joachim Wuerker
    Prodigy 120 points
    Hello Joachim. I should have looked closer at the ADS1220 block diagram. Did do this after your post. It shall be well saved in my memory banks.
    Thank you for the suggestion. Likewise, the ADS124S08 would be an excellent alternative in a few ways. That is a neat part.

    Regards,

    Tony