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ADS131M06: K-Thermocouple + cold junction compensation

Alen Barisic
Intellectual 1300 points
Part Number: ADS131M06
Other Parts Discussed in Thread: SN6501, ADS124S06, ADS124S08, ADS131M08, LMT70

Hi,

I am looking for a simple ADC to be able to acquire several K thermocouples. Overall, the finished device should be able to evaluate 60-70 TC's. Measuring channel groups with 10-20 measuring channels must be used, which also have to be electrically isolated. For two measuring channels a cold junction compensation is needed.

After researching it I found the ADS131M06.

I could measure a total of 6 + 6 TC's and 6 cold junctions with 3 ADC's. I would then galvanically separate this group. This circuit applied six times and I would have 72 temperature measurement channels.

• Are there ADCs that are more suitable? (min. 16 bit, 1kSPS)
• I read about a "small, accurate" temperature sensor. -> Which TI temperature sensor is suitable for cold junction compensation? I don't want to calibrate this sensor.
• We will do galvanic isolation with the SN6501 and a not yet selected digital isolator
Thanks for your support,
Alen
  • 0
    TI__Guru 63915 points

    Hi Alen,

    That's a lot of AD131M06 devices!  If you don't need the throughput that the simultaneous sampling gets you with the M06, then consider the ADS124S06 or ADS124S08.  They're very popular devices for thermocouple measurements and include some helpful diagnostics and biasing options specifically targetted towards sensor acquisition which will benefit the RTD or thermistor based CJC measurements.

  • 0
    TI__Guru* 97775 points

    Alen,

    I think the ADS131M06 would be a good device for this application. You could also use the ADS131M08 if you need a couple more channels. However, Collin is correct that the ADS124S06 and ADS124S08 are also good for consideration. They have better noise performance, but these devices use a multiplexer on the front end, and are significantly slower (up to 4kSPS max for the ADC).

    For cold-junction measurement, there are plenty of options. You could use an RTD for the cold-junction (though three separate cold junctions might be difficult to route). I think LMT70 would be a good option. It's an accurate temperature sensor and it's small enough to be placed near any cold junction.

    As for the galvanic isolation, I would guess that you'd need the something like the IS07742 for the communication.

    If you have more questions about the temp sensor and the isolation, I would post in the respective forums for questions about those.

    And just in case you haven't seen it, we do have an application note on the use of thermocouples:

    http://www.ti.com/lit/an/sbaa274/sbaa274.pdf

    Joseph Wu