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Original question:

ADS1118: internal temperature has a 2-degree offset

ADS1118: will heat up about 1.2C

Howard Zou
Genius 12365 points
Part Number: ADS1118

Hi,

my customer read ADS1118 internal temperature, and find that it will gradually heat up after power up. The temperature will go up about 1.2C than the initial value after several minutes and then be stable. It's not caused by noise because it's gradually heat up.

The schematic is shown below and no heat source is found. ADS1118 itself will not heat up according to the datasheet. R41, R42 is not soldered so only AIN0, AIN1 is used.

I wonder where may the heat from?

The board is totally only connected through external circuit through wire shown below, so the heat should not be generated from external circuit.

With a fluke multimeter, it shows that the temperature of ADS1118 really goes up about 1.2C

  • 0
    TI__Guru* 93715 points
    Howard,


    Based on the schematic, there isn't much on the board that would cause extra heat dissipation. The only things that I can think of that would cause that kind of heat dissipation are a damaged device or some sort of over-voltage that causes the device to draw extra current.

    If you go into the ADS1118 datasheet, the RGS package junction to case thermal resistance of about 50°C/W. For the case temperature to rise by 1.2°C, this means that the device would need to dissipate 24mW of power. At a supply of 3.3V, that means the device would need to pull an extra 7mA.

    There are two things that you can check. First, the ADS1118 has an accurate temperature sensor on board. I would first take a series of readings to verify that the temperature is rising in the device. Second, I would put in an ammeter and measure the current to the device. Measure the current going into VDD, but I would also measure the current coming out of the ground. Of course, the two should match. However, if the current coming out of the ground is higher, there may be an over voltage coming from another pin. Check the digital input pin voltages to make sure the VDD voltage matches with the microcontroller voltage (and that the grounds are the same). Also check to see that the inputs are biased to the correct supply midpoint voltage.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Genius 12365 points
    Joseph,
    thank you.
    Both current go in VDD and go out to GND are the same, 160uA.
    The reading of the temp sensor inside ADS1118 also tell that the temperature is rising.
  • 0 in reply to Howard Zou
    TI__Guru* 93715 points

    Howard,


    I'm not sure where the temperature rise is coming from. 160uA isn't enough power to cause a 1.2°C rise in the device temperature. You'd need almost 50 times more current to create that kind of a temperature change. The alternative is that the thermal resistance of the device is really 2300°C/W. I don't see that the current can come from this normal supply current. I'd look to other sources of current that could provide the heat.

    In my last post, I suggested that the current might be coming in from a pin other than VDD. Is it possible that there is extra current coming in from the digital lines or analog input lines? Is it possible that there is something else on the board that is heating the device?

    I've seen the schematic, and I don't see anything that might be a problem. The protection diodes can only be placed in one direction, so they shouldn't be drawing extra current unless they are damaged.

    Do you have access to a close-up photo of the board? Maybe there's a solder bridge of some other bad connection on the board itself. Have they verified the resistor values on the board? Do you have a copy of the PCB layout?


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 93715 points
    Howard,


    I haven't heard from you for a while. Were you able to find a solution to this heating problem? As I indicated earlier, I think heating is coming from an external source, or the current is being driven through a different pin through one of the ESD diodes and out another supply or ground pin. The current of the device is would need to be almost two orders of magnitude to drive temperature up by 1.2°C.

    I'll close this post for now, but if your customer continues to have problems, post back and we can continue debugging. Go through my last post and check with the customer for responses.


    Joseph Wu