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ADS1118: calibration with fluke 753

Howard Zou
Genius 12365 points
Part Number: ADS1118

Hi,

my customer is using ADS1118 for thermocouple temperature measurement.

They use fluke 753 to act as a thermocouple to calibrate their equipment. FLUKE 753 users manual link is as below:

Source mode page 54 describes thermocouple simulation of 753.

They will observe around 2C error the temperature measured from ADS1118 if 753 is considered 100% accurate.

1. What may cause this error?

2. If they change the pull up and pull down resistor in Figure50 in datasheet from 1Mohm to 2Mohm, the error could be reduced to 1C. Why increasing the pull up and pull down resistor could help?

3. Are there any risk to increase the value of pull up and pull down resistor?

4. Do we have suggested calibration method for thermocouple temperature measurement circuit?

  • 0
    TI__Guru* 97585 points

    Howard,

    It's hard to say where the error comes from because the error sources are generally very small. A 2C error in the temperature using a K-type thermocouple is the equivalent to 82uV of error. Is this error at room temperature or is this across a large range? What thermocouple type are they using? I'll just answer your questions, and we can discuss the error through the answers to the questions.

    1. As I mentioned, there could be several errors. This could be part of the offset error or gain error of the ADC. It could be an error with the cold-junction compensation, it could be the biasing current going through the resisitive leads of the thermocouple.

    2. If the change in the pull-up/pull down resistor causes a change in the error, then this error could be the current running through these resistors reacting with the thermocouple leads. However, this current is rather small, and the resistance of the leads is also small. If the customer uses a precision multimeter, they could read the voltage and see how much it changes for each configuration.

    3. I don't think there's a risk in raising the resistor values to 2MΩ. With lower biasing current, this might change the biasing point a little and it would lower the burnout voltage a bit, but it shouldn't affect the operation.

    4. I don't have a specific calibration method for thermocouple measurement. However, I would check several things. First, be able to make measurements with a precision volt meter to compare the values to the ADC measurements. Second, verify that the cold-junction compensation measurement is done properly. This will include measuring the temperature of the device (if they're using the ADS1118 internal temperature sensor for CJC). They should also verify that the thermocouple leads and the cold junction are at the same temperature.

    The Fluke 753/754 does have some some error specification, but for the most part, they are about 1°C or below.

    Just to make sure, we do have some application notes on the ADS1118 and thermocouples that the customer can review. Here's a guide to thermocouples and a couple of application notes for the ADS1118 and thermocouples:

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

    http://www.ti.com/lit/an/sbaa189/sbaa189.pdf

    http://www.ti.com/lit/an/sbaa354/sbaa354.pdf

    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Genius 12365 points

    Joseph,

    thanks.

    I have a question about this article:

    Resistor values are generally chosen to be from 500 kΩ to 10 MΩ depending on the input current. Different ADCs have different magnitudes of input current. If the resistance is too high, the biasing current becomes too small compared to the ADC input current of the resistors.

    Why the biasing current smaller than ADC input current is not okay?

  • 0 in reply to Howard Zou
    TI__Guru* 97585 points

    Howard,


    Generally, we use the biasing resistors to keep the thermocouple near the mid-supply (VDD/2). If the biasing resistors are the same and the input current to the ADC small in comparison to the biasing current, then the thermocouple will stay near mid-supply.

    However, if the input current to the ADC is large compared to the current through the resistors then two resistors would not set the biasing like a voltage divider near mid-supply. The current pulled out of resistors would cause the bias point to be pulled toward ground (or supply if current comes out of the ADC).


    Joseph Wu