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TMS320F2812: ADC reading at cold temp

Aaron Spaete
Expert 4607 points
Part Number: TMS320F2812


Team,

My customer has a design that has been out in the field for a year.  They have a built in test function to simulate the signal coming into the ADC.

They output to 5V or drive to full scale.  The clamp on the ADC limits the current to < 1mA.  They are getting a 1.5% fallout rate when doing this test at cold temps.   The ADC reads absolute 0.

Any thoughts?

Regards,

Aaron

  • 0
    TI__Guru 62975 points
    Aaron,

    Is there a reason for forcing 5V when the full-scale input range is 0V to 3V? What is the voltage on the ADC pin when measured externally? Is the device able to recover after removing the 5V and initiating a full power-off + power-on cycle?

    The current clamp will help to avoid permanent damage, but I think it still leaves the ADC circuitry susceptible to latchup when forcing in such a high voltage. As far as I know, the device pins are not 5V tolerant.

    -Tommy
  • 0 in reply to tlee
    TI__Expert 4607 points

    Tommy,

    During this test, the ADC input is driven by an op-amp (I will tell you the part number off-line) that is powered off the same supply as the ADC, which is 3.3V. The op-amp output will be saturated to nearly the supply voltage.  I believe the ADC inputs are diode clamped to the ADC supply, so we would not actually be driving current through the clamps in this case. We designed it this way to prevent the clamps from ever being able to conduct since the ADC input voltage can never get high enough.

    Regards,

    Aaron

  • 0 in reply to Aaron Spaete
    TI__Guru 62975 points

    Aaron,

    That should be fine if the op-amp shares a supply with VDDA.  Do you know the temperature range where this happens?

    Reading back a conversion result of 0 sounds like perhaps something in the digital domain is not working right.  Is there test code that checks other functions of the device?

    Would you happen to have a code snippet for the ADC test initialization and configuration?

    -Tommy