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AD1118 Internal temperature deviations

Kresimir Besedic
Prodigy 20 points
Other Parts Discussed in Thread: ADS1118

Hello,

I am developing a thermocouple (T) measurement device.

The AD1118 is connected according the data sheet. Voltage supply 3.3V , MCU AT2580 SPI 5V.

The problem which I am facing now is the internal temperature sensor deviation.

I get always a measurement which is 8-10% higher than the real temperature of the chip/housing in the range up from 0.0C. It is compared with a Fluke thermocouple device and a Infrared thermometer which both showing the same value (+/- 0.1C)

eg.: ADS1118 is sending 0x1064 or 32.78125 C ( readout on the DOUT pin with a scope) but the real temperature on the housing (ambient temperature) is 30.3C when the ADS1118 should sent 0x0F26 , more or less.

The configuration bytes sent to the ADS1118 are 0x0A7A and return is 0x0A7B.

The SPI communication is very clean and disconnected thermocouple is showing 0x7FFF.

Is there any kind of calibration needed? I could not find anything about!

Due the fact we are close to the deadline for finishing our project, every help and suggestion about how to get the correct internal temperature measurement will be appreciated.

Thank you in advance

Kresimir

  • 0
    TI__Guru** 113765 points

    Hi Kresimir,

    It appears like your configuration and temperature conversion is correct.  Keep in mind that the actual temperature sensor value should be measuring the Cold Junction area and it may differ from the case temperature.  The ADS1118 is low power and has very little self heating, so it is possible that the copper area of the board is warmer than the case temperature. If you want, you can send me your layout and I'll take a look at it.

    As far as calibration, yes that is a good idea. There are many calibration methods.  The simplest is two points. To calibrate you will need a standard. 

    If you haven't read through this document yet you may find it helpful:

    http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sbaa189&fileType=pdf

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hi Bob,

    thank you very much for the fast response.

    I already have downloaded this document . It looks like  that you are right and there is difference between the case, pcb and ambient temperature.

    When I measure the PCB temperature close around the ADS1118 it is always lower then the temp on the case.

    PCB temp: 33.0-33.4

    ADS1118 Case temp: 34.3C

    Internal temp sensor : 36.3125 C

    I any case the ADS1118 is the most warmed up place on this part of the pcb ?!?

    I am using the MSOP 10 Package.

    In past projects I have used the LTC2480 from Linear , but the sampling rate is to low so I decided to use the ADS1118 which is very easy to implement. But with the temp sensor of the LTC2480 very accurate internal measurements are possible with minimal deviations compared to the "outside" temperature.

    It looks like we will have to redesign our pcb with more GND copper around.

    Will sent you pcb pictures soon.

    Best regards

    Kresimir