• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 48 subscribers
  • Views 893 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1248, Grounded thermocouples

MicroM
Intellectual 260 points

Other Parts Discussed in Thread: ADS1248

Hi everybody I am trying to measure K or J type thermocouples with ADS1248, some of this thermocouples are "Grounded" and some other are "Ungrounded". Also i don't know which of them are Grounded or Ungrounded! I know that, for Ungrounded thermocouples I must use "Vbias" (Vdd = 5v, Vss = 0) For measuring, is there any differences between Grounded or Ungrounded thermocouples? thanks,
  • 0
    TI__Guru** 101175 points

    Mahdi,

    Grounded and ungrounded refer to the electrical connection of the thermocouple protective sheath. Grounded means that the thermocouple is electrically connected to the probe wall. This makes for a faster response time because of better heat transfer, but it's not electrically isolated. Ungrounded means that the thermocouple is not electrically connected to the probe wall. This gives a slower response time, but it's electrically isolated from the circuit. There are also exposed thermocouples without any protection.

    There shouldn't be much of a difference in using either to measure temperature, but with grounded thermocouples, you have to consider that there may be an electrical connection to the measurement and you have to watch for ground loops in the system.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 260 points
    hi Joseph thank you for you reply you mean for both Grounded and Ungrounded TC we must use Vbias connected to the negative pin of TC? in datasheet on page 29 focus on "Ungrounded thermocouples" sometime in my board (my board has 4 channel) when i connected different kind of K/J TCs suddenly i see an offset about 1.5mv i try to test with different kind of TC, i think when i connect a grounded TC this offset happen. (be notice that when the this effect on all channels)
  • 0 in reply to MicroM
    TI__Guru** 101175 points

    Mahdi,


    The datasheet does use the term "ungrounded thermocouple" on page 29 as you describe. Really what the datasheet should have said was "unbiased thermocouple". The VBIAS is used to establish a common mode voltage for which to make the measurement. The ADS1248 can't make a measurement of a floating input voltage.

    As for the offset that you see, I'm not sure where it's coming from. It could one of a few things. I would make sure that the burnout sources are off and the IDAC currents sources are off. Those currents, reacting with series resistances may make add an error. If you are using a grounded thermocouple, make sure that the outer casing is not connected to anything that could cause a ground loop voltage. This may cause currents to form that could also create a similar error.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 260 points

    Hi Joseph

    thank you for your answer.

    i test more and post later for offset.

    best regard

    mahdi