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ADS1118: 1

Igor Sandler
Prodigy 10 points
Part Number: ADS1118

Hello,

For lab check of part: ADS1118 Ultrasmall, Low-Power, SPITm-Compatible, 16-Bit Analog-to-Digital Converter with Internal Reference and Temperature Sensor

I will appreciate your answers for the following question:

1. The component ("part - ADS1118") is cold junction compensation - should the component be kept at constant temperature as possible ? e.g - connecting component case top to heatsink via thermal pad

2. If the component is not kept at constant temperature - will it show false/non physical temperatures values ?

3. Do you have any thermal experiment reports of this component, tested when implemented on PCB

4. In the DS at table 4: 14-Bit Temperature Data Format, as per my understanding the showen temperature is interpolation between two values:

e.g - if measured 7°C will it show 5°C or 10°C ?

5. Any specific recommendations when the component is implemented in rapid changing environment - components ambient temperature is changed very fast

Hope to hear from you soon,

Igor

  • 0
    TI__Guru** 113765 points

    Hi Igor,

    Welcome to the E2E forum!  Check out the following TI design:

    https://www.ti.com/tool/TIPD109

    I think most of your questions will be answered in the design documentation.  As to specific answers, it is not totally clear what you are trying to do.  If you need additional theory regarding thermocouple measurements, take a look at A Basic Guide to Thermocouple Measurements.

    The ADS1118 is designed for measuring the cold-junction of a thermocouple.  The cold-junction itself should be isolated and stable.  Air currents and PCB temperature variations can affect the measurement.  The internal temperature sensor can measure from -40 to 125 deg C where the temperature resolution is 0.03125 deg C per bit of resolution.  The temperature sensor accuracy is listed in the datasheet in the Electrical Specifications on page 7.

    If you are not interested in thermocouple measurements and are just wanting PCB temperature, then TI offers a complete line of temperature sensor products.

    https://www.ti.com/sensors/temperature-sensors/overview.html

    Best regards,

    Bob B

  • 0
    Prodigy 10 points

    Hello Mr. Benjamin,

    Thanks for the prompt response, can you please explain more specifically on what assumptions (experiment, calculations etc.) your answer based ?

    I refer to:The ADS1118 is designed for measuring the cold-junction of a thermocouple.  The cold-junction itself should be isolated and stable.  Air currents and PCB temperature variations can affect the measurement. 

    Regards,

    Igor

    Thermal engineer

  • 0 in reply to Igor Sandler
    TI__Guru** 113765 points

    Hi Igor,

    I'm not entirely sure what you are asking for or about.  Are you asking specifically about the ADS1118 or the TI-Design that I referenced.  As far as the internal temperature sensor of the ADS1118, this will actually measure die temperature.  In most cases the ambient will be very close to the package/die temperature as the ADS1118 has very little self-heating.

    As far as an actual tested thermocouple application, the TI-Design referenced in my previous post covers all aspects of the design and testing procedure.  On the same landing page as the TI-Design is a user guide that explains the design in detail.  You can find this document here as well.

    It is still not clear what you are attempting to measure (thermocouple, ambient temperature, PCB temperature, etc.) or what is prompting the questions.  The ADS1118 is an ADC with an internal temperature sensor.  The ADC can measure the thermocouple voltage but will require compensation.  You could use many different methods to measure the cold-junction, and one of those methods is to use the internal temperature sensor of the ADS1118.  All computation for the cold-junction will need to be done by an external processor as the compensation is not done within the ADC itself.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 10 points

    Hello Mr. Benjamin,

    Regarding my former questions about component ADS1118, Ill appreciate your time answering below questions:

    1. In your previous answer you have mentioned that the component should be isolated from the environment - what kind of insulation materials/techniques you can recommend?

    2. Where is located the temperature sensor in ADS1118 (bottom case side, in the die, top case side)?

    3. What temperature the sensor is measures (Junction temperature, case temperature)?

    Thank you for your time,

    Igor

  • 0 in reply to Igor Sandler
    TI__Guru** 113765 points

    Hi Igor,

    The ADS1118 temperature sensor's primary application is for measuring cold-junction for thermocouple measurements.

    Igor Sandler said:
    1. In your previous answer you have mentioned that the component should be isolated from the environment - what kind of insulation materials/techniques you can recommend?

    When measuring the cold-junction temperature the point of connection should be thermally isolated from other temperature gradients that may exist on or around the board connection.  This does not refer to a specific material, but rather attempting to maintain a constant temperature in the cold-junction.  An layout example is shown in the application note I referenced in my previous post.

    Igor Sandler said:
    2. Where is located the temperature sensor in ADS1118 (bottom case side, in the die, top case side)?

    The temperature sensor is located in the die.

    Igor Sandler said:
    3. What temperature the sensor is measures (Junction temperature, case temperature)?

    The temperature sensor measures the die temperature.  As the device is low power, there is little self-heating and the package temperature is very close to the die temperature with the RUG package being the closest to PCB, package and die temperature equivalent.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 10 points

    Hello Mr. Benjamin,

    Appreciate your time answering my questions. Regarding your answer to my 1st question: "An layout example is shown in the application note I referenced in my previous post."  - can you please point me to the page number.

    Another question/request - Currently we have implemented the ADS1118 components on a PCB (Temperature measuring card to which we connect Thermocouples). From multiple uses of the card it seems that we have some issues that our card provides false/non physical readings. Is it possible to share with you the card components placement (via your email) to receive your insight how to improve our card?

    Regards,

    Igor

  • 0 in reply to Igor Sandler
    TI__Guru** 113765 points

    Hi Igor,

    The layout for the TI Design TIPD109 is on page 16.  The actual gerber files are located on the TIPD109 reference design landing page or can be downloaded from this link.

    It is not clear what you mean by "false/non physical readings".  Can you give an example?  Usually the biggest issue with bad data is due to the thermocouple wire acting as an antenna where EMI/RFI can be picked up.  After reviewing the above material and you still have questions let us know and we can contact you through the email you provided when you signed up to E2E.  As an FYI, I do not normally support this device, so you would be contacted by someone else.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 10 points

    Hello Mr. Benjamin,

    Thank you for providing the information:

    The layout for the TI Design TIPD109 is on page 16.  The actual gerber files are located on the TIPD109 reference design landing page or can be downloaded from this link. - ill forward this information to our relevant engineer.

    Regarding:

    It is not clear what you mean by "false/non physical readings".  Can you give an example? - "A picture is worth a thousand words", but ill try to explain as best as I can.

    Assume 4 TCs lines connected to 2 ADS1118 components. all the TCs are measuring transient environment, they supposed to show almost identical temperature profile. But each pair shows different profile (each pair is connected to different ADS1118). We tried to understand how that could happen:

    1. Unwanted external heating (heating power surge) of one of the ADS1118 

    2. Unwanted external heating (heating power surge) of one of the TCs connectors - caused creation of temp. difference between the TCs connector and the die (cold junction). which created voltage difference....

    Usually the biggest issue with bad data is due to the thermocouple wire acting as an antenna where EMI/RFI can be picked up - can you please give an example or explanation. Moreover, does the ADS1118 have compensation for that or it should be implemented on the PCB ?

    Best regards,

    Bob B

  • 0 in reply to Igor Sandler
    TI__Guru** 113765 points

    Hi Igor,

    When you have multiple thermocouples connected to a single ADC it is difficult to determine precisely the cold-junction temperature.  If you heat the ADS1118 alone, and the temperature of the cold-junction differs from the ADC die temperature, then your measurement will be inaccurate.  In a similar way, if the cold-junction temperature changes apart from the ADS1118 die temperature then the measurement will also be inaccurate.  So this would be expected behavior.  The end goal is to maintain a cold-junction measurement accurately.  This may require that a different sensor is used to measure the cold-junction more precisely.

    As far as EMI/RFI pickup, think in terms of radio and the wiring acting as the antenna.  The ADC will try to measure any applied voltage, and with high-frequency content will appear to be an offset for a near DC conversion result.  So some method of preventing the EMI/RFI signal from entering the ADC inputs is required.  Often ferrites in combination with an RC input filter to the ADC can be sufficient.  Using shielded wires with the shield properly terminated  is another method.

    Best regards,

    Bob B