MAX31855 cold-junction compensated thermocouple-to-digital converter

Posted on Saturday, April 23rd, 2011 in app notes by Ian

Drone writes:

Here is another MAXIM unobtainium installment

The new MAX31855 is a rather nice cold-junction compensated thermocouple-to-digital converter. It takes the likes of the MAX6675 to the next level. This part is supposedly going to become available around the end of June 2011. From the product page:

“The MAX31855 performs cold-junction compensation and digitizes the signal from a K, J, N, T, or E type thermocouple. (Contact the factory for S and R type thermocouples.) The data is output in a signed 14-bit, SPI™-compatible, read-only format. This converter resolves temperatures to 0.25°C, allows readings as high as +1800°C and as low as -270°C, and exhibits thermocouple accuracy of ±2°C for temperatures ranging from -200°C to +700°C for K-type thermocouples.”

There is a different part optimized for each type of thermocouple junction. For-example, the MAX31855K part is optimized for type-K thermocouples. I’ve seen these up for pre-order at $8.19 in unit quantity (ouch).

Via the contact form.

This entry was posted on Saturday, April 23rd, 2011 at 6:25 am and is filed under app notes. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.

8 Responses to “MAX31855 cold-junction compensated thermocouple-to-digital converter”

  1. MichaelZ says:

    I ordered some samples. Wont get them until July. This chip can read below 0 deg C which the MAX6675 could not do. Also it is a lot cheaper than the MAX6675.

  2. Ryan says:

    Just got my samples of this last week. Trying them out an working on a breakout board and Arduino code. Seems to be working good so far.

  3. Rogier Wolff says:

    My sample now works. It used to have a 3 to sixty degree sawtooth on the temperature reading. The solution was to put a capacitor on the temperature probe.

    • raimilson says:

      Could you share the value of the capacitor you used? I always used the MAX 6675 and now I need to read under 0°C temperature and moving to MAX31855 but no luck until now, I only got messed values. any help would be apreciated.

    • yoppy says:

      I also experienced strange results. When I used a short TC wire ( +/- 20 cm), the results were ‘quite good’, although sometimes there were intermittent unexpected readings. Even worse when I used a long TC wire. You say that you put a capacitor on the temperature probe. Do you mean connect the T+ and T- with a capacitor? Any kind of capacitor?

      Thank you.

  4. Roger Wolff says:

    Yes, I could. :-)

    I used 10uF.

    Maxim technical support says: “The largest value I have used for filtering is 10nF.”…..

    So there are a few possibilities: I’ve added a super-large capacitor that has a relatively high ESR, so it’s effectiveness is lower than a 10nF one, because especially the ESR counts. Or I have a very noisy environment that requires a much bigger capacitor.

    I don’t think my capacitor has a high ESR. I think they are these:
    I’ve just done some reading-up on the ESR of these (Y5V) capacitors, and it seems that this would be on the order of milli-ohms (about 10). -> This is NOT a factor.

    So my 10uF should work about 1000 times better than a 10nF capacitor.

    I still have the sawtooth, only it’s only 0.75 degrees high, so it is acceptable.

    My layout is not ideal: I have a converter board stuck onto a development board. So I expect that a proper layout would improve the situation a bit. But if you’re designing boards, design a 0805 footprint near the chip across the T- and T+ pins…. You can always leave it empty. :-)

    • Raimilson says:

      Thank you very much my friend, in my case the 10nF suited better, now I am able to read the ADC output correctly. Also there is a important thing, this MAX31855 cannot handle grounded Thermocouple, they should be ungrounded, this was another thing that I figured out.

      • Roger Wolff says:

        Oh yes. That was the first thing that Maxim technical support asked me. It starts to seem that this is common.

        The MAX31855 needs the thermocouple to float because it needs to put it at a voltage to te test for the ground-fault. Furthermore, to measure low temperatures (lower than room-temperature) , the voltage at the T+ pin will be lower than the voltage at T-. So if you ground T-, that voltage will be outside the powersupply range: difficult to measure.

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