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ADS1120: temperature measurement using thermal pad

Stani Bell Ngoufack Tchoufack89
Intellectual 2020 points

Part Number: ADS1120

Hello,

my customer is currently working a project with ADS1120 (VQFN Package) and they also to plan to measure temperature using the thermal pad of the device. the engineer would like to know if the thermal pad is floating or if it's internally connected? can they connect the pad to the metal housing (it's not connected to any potential) of their equipment in order to measure its temperature?

Also the device is used to measure voltage on a plug contact socket; however the engineer saw a deviation of approx. 150uV between what was captured by the ADC and the same voltage measured using a voltmeter. now he wants to know if its appropriate to use this device for this kind of measurement and if there are any care abouts to take into consideration.

Best regards,

Stani

  • 0
    TI__Guru** 113765 points

    Hi Stani,

    The ADS1120 datasheet recommendations in the Pin Functions table on page 4 for the thermal pad are "Do not connect or only connect to AVSS".  If the pad is soldered to an area that has no electrical potential, then the pad would be essentially floating electrically.  However, in many cases a metal housing is connected to earth ground which is not floating.

    I'm having a difficult time envisioning how this connection would actually work.  For the ADS1120, the center pad is most important for package stability as opposed to power dissipation as the ADS1120 is considered a low power device.  For the ADS1120 QFN package to be stable, the pad must be adequately attached.  It is hard for me to understand how the package would be adequately attached to such a large thermal mass and how the remaining connections to the ADS1120 would be made.  QFN packages are very susceptible to damage if not properly attached.  So at this point I would not recommend the usage of the device this way without fully understanding how the ADS1120 would be connected.

    To me it would make more sense to use a thermistor or RTD to attach to the metal housing and measure the temperature that way using the ADS1120.

    As far as a voltage measurement, the ADS1120 is well suited, but there are some considerations.  If the PGA is enabled, then the input must be within the proper common-mode input range.  If the measurement is ground referenced, then the PGA must be disabled and bypassed.  Can you give me more detail as to the ADS1120 device register configurations and the voltage applied?  Is there a schematic?

    Best regards,

    Bob B