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Diode protection on ADC pin for 28377D experimental kit

Amtech
Genius 4545 points

Hi All,

I am using 28377D experimental kit. As per data-sheet absolute maximum rating for VDDA is -0.3V to +4.6V. On experimental kit the diode (on ADC pin) that you choose has 0.8V forward voltage drop.

In case if my ADC voltage becomes -0.3V to -0.8V then there will be no protection and my DSP will be damaged.

I want to know what is the reason to choose that diode?

Am I thinking right or there is a different reason to choose this kind of diode.

Thanks and regards,

Maulik Timbadiya

  • 0
    TI__Guru 60865 points
    Hi Maulik,

    You are essentially correct - these diodes will not provide full protection to the ADC inputs in all circumstances.

    Keep in mind that the ADC inputs also have internal clamping diodes. Practically, damage will not occur to the pins unless the internal clamping current exceeds about 20mA (this is for a given pin, and also for the sum of all pins combined). Since there is also usually some series resistance driving the ADC pin, the external clamping diodes will eventually turn on before the pin is destroyed in many circumstances.

    If you want the best protection for the inputs on your application board, do try to use protection diodes with very low forward voltage drop.
  • 0 in reply to Devin Cottier
    Guru 19965 points
    Hello Devin,

    Are you saying that -0.8V at the ADC pin is ok as long as the input current doesn't exceed -20mA?

    Stephen
  • 0 in reply to stevenh
    TI__Genius 13276 points

    Stephen,

    Different device, same answer. :)

    I'm afraid we have to advise that both the voltage and current requirements be met.  -0.3V is an absolute rating in the DS and exceeding this can cause unpredictable results or even damage the device.

    Regards,

    Joe

  • 0 in reply to Joe Prushing
    Guru 19965 points
    Not the answer I was look for.

    Why can the uC handle a much larger positive over-voltage, i.e. 4.6-3.3 = 1. 3V, and handle a comparable negative under-voltage, i.e. -1.3V?

    Stephen
  • 0 in reply to stevenh
    TI__Genius 13276 points
    More than a diode below ground and you'll start current injecting into the ground plane which can raise the entire ground plane of the device. That can cause a lot of unpredictable behavior.
  • 0 in reply to Joe Prushing
    Guru 19965 points
    For my situation, the clamping diodes I am using allow the voltage to go as low as -0.5V. I also limit the current into the ADC pin by using a 499Ohm resistor. Therefore, the ADC current should be limited to approximately 400uA (i.e. (0.3-.0.2)/500 = 400uA). I wouldn't think 400uA would have any affect on a large ground plane.
  • 0 in reply to stevenh
    Genius 4545 points

    Dear stevenh,

    My personal suggestion is : Please do not use diode which has 0.5V forward voltage drop. As -0.3 to -0.5 V  may cause unpredictable behavior.

    Regards,

    Maulik