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ADS1118 chip failure

Jonathan Neuendorf
Prodigy 70 points
Other Parts Discussed in Thread: ADS1118, CC2530

Hello,

So I have a PCB with ADS1118's each reading 2 thermocouples and communicating via SPI with a CC2530. I had 4 prototypes that I know worked well Monday morning. When I tried testing them on Monday afternoon, they all had the same issue. The ADS1118 output no longer responds to the thermocouples. They constantly read either a near negligible reading or a reading that corresponds to around 3 degrees C. The on-board temperature sensor works just fine.

I checked the SPI communication and was able to verify that the ADC's were receiving the proper command, and verified that the values that I was receiving on my PC correlated to the values sent from the ADC's. Furthermore, I checked the inputs to the ADC and verified that the thermocouples do operate properly. They just aren't being read properly.

So at this point, I've concluded that something is wrong with the ADC's themselves. However, this is 4 separate units that all went bad at the same time. Even worse, I tried replacing the ADC's on one with some spares I had in my desk. Unfortunately the problem persisted. What am I missing? Could something have happened to fry the ADC portion of all of these chips at once? 

Any suggestions as to what to check or what the issue might be would be greatly appreciated. 

Thanks,
Jon 

  • 0
    TI__Guru* 97465 points

    Jonathan,

    If the internal temperature sensor still works, then we know that the ADC is still converting. Now we need to check the MUX and the PGA.

    Are you able to measure anything else besides the thermocouples? Can you drive them with any other voltages for measurement? Even taking the inputs, one to either rail would give a full scale measurement. You can also check to see if you have any inputs shorted to each other or either supply.

    You can also read back the configuration register to make sure that you have everything configured the way you think it should be.

    I had a couple of other questions. What input voltage gives you a 3C reading? Is it close to 0V? Is there any noise or variation on this measurement? You mention that you replaced one of the ADCs and the problem persisted. Do you mean that the new ADC was immediately damaged? Could there be a problem on the board itself?

    When you first mentioned the problem, I might have guessed that the part was damaged by an ESD event, but generally the ADS1118 is pretty resistant, and is rated to 4kV for HBM ESD.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 70 points

    So after a week of ripping my hair out... I finally discovered the issue... in my Matlab code I converted the ratio received from the ADC to Volts. I put that value directly into a lookup table function whose input was mV.... So the thermocouples were responding, but I couldn't see it because it was 1000x less than it was supposed to be...

    Your suggestion of applying a voltage to the thermocouple inputs was what did the trick. I was able to apply enough voltage, about 120mV, to see that what I was reading was 1000x less than what I should've been. I really should've thought of doing that on my own... too late. I think the 3C reading is the floating voltage when I have a bad connection to the thermocouples. It's much much higher when I'm not shrinking it by 1000...

    So thank you very much for your time.

    Regards,
    Jon