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ADS7828E fails and draws high current

Michael Adamson
Prodigy 30 points

I am using ADS7828E with Vdd at 3.3V as shown in the attached image. After some amount of successful run time, the ADC will inevitably fail and output a constant value - not the same output every time. When this happened originally, the Vdd pin dropped to 2.2V drawing 100mA. After lowering the resistor in series with the supply to 5 ohm and adding an additionally bypass cap, we thought the problem was solved. However, the unit still inevitably fails, but now the voltage drops to 2.8V still drawing over 100mA.

We are considering removing the 5 ohm resistor in series with the power supply, as well as changing the bypass cap at Vref to somewhere between 1uF and 10uF. I'm not convinced that either of those will solve the problem.

Are there any known scenarios that would cause this issue?

  • 0
    TI__Expert 6925 points

    Hello Michael,

    Since improving supply decoupling seems to be helping, this might be related to input rating violation. When inputs have a voltage outside the absolute limits, ESD cells activate connecting the input to the supply. If this happend many times, it is normal for the device to get damaged.

    They typical scenarios for this are either input transients or having voltages applied to the inputs during power-up and power-down. For the second case, during power up or power down, it would be good to check the inputs and supply pin with an oscilloscope.

    By the way, what is the supply of the amplifiers at the ADC inputs? Or are the ADC inputs just connected to a PCB header? Or, what are the inputs connected to?

    Regards,
    Rafael

  • 0 in reply to Rafael Ordonez
    Prodigy 30 points

    What we think really helped was putting the 0.1uF cap between Vref and ground. In earlier designs, Vref was just grounded - but that still did not entirely solve the problem. Can the Vref circuit contribute to current problems like I've described?

    I do not believe that power up/down is the problem. Resetting the board will bring the ADC back to normal function, but the issue will come back eventually. We can do a "band-aid" fix and put an auto reset feature in, but we really want to get to the root of the problem.

    The ADC inputs are connected to various sensors, and are pulled up to Vdd and pulled down to ground where necessary.

    We will begin investigating the behavior of the ADC given inputs outside of rating

  • 0 in reply to Michael Adamson
    Prodigy 30 points

    There is a 3-4 foot long cable leading from the sensors to the header which traces to the ADC. My unit here responds as one would expect given an overvoltage and the input - the current gets routed through the supply which causes Vdd to rise and the supply current to fall. As soon as the input is reduced within the rating, Vdd returns to normal. This operation is the opposite of our issue.

    As I said before, resetting the board causes the ADC to function normally again for whatever length of time until it decides to fail. This leads me to believe that the board isn't damaged.