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AM335x ADC Clamp

JasonHaedt
Genius 3320 points

If I use an external reference, can I use the ADC input pins diodes to clamp the signal?  I would obviously have a 2k impedance in series.

Thoughts?

-Jason

  • 0
    Genius 3320 points

    Hello TI,

    I have not seen feedback on the AM335x ADC.  In addition, I wanted to verify:

    Is there any issue with using a 1.25V reference (VREFP) and ADC power will be the nominal 1.8V?  All the evaluation boards use these power buses connected together at 1.8V.

    -Jason

  • 0
    TI__Prodigy 40 points

    The ADC input pins diodes are not intended to be used for DC clamping the signals; Rather for ESD protection. They clamp at about 2.75 V (With a 1.8 V supply) which is higher than the max recommended input voltage which is 2.1 V. The 2K in series helps limit the diode current to a safe value which should not exceed 25 mA.

  • 0 in reply to JasonHaedt
    TI__Prodigy 40 points

    No issue with using 1.25 V VREFP. However, your input signal dynamic range should be within VREFP-VREFN.

  • 0 in reply to Maher Sarraj
    Prodigy 30 points
    You say the ADC input diodes are not intended to be used for DC clamping, but as long as the current is limited to 25mA then it is ok to use them for continuous DC clamping?

    According to the AM335x datasheet "Steady State Max. Voltage at all IO pins = -0.5V to IO supply voltage + 0.3 V". Can I exceed the 0.3V as long as I limit the current to under 25mA DC, and as long as the IO supply voltage doesn't exceed 2.1V?
  • 0 in reply to Jaap Overtoom
    TI__Prodigy 40 points

    Clamping the IO pins more than 0.3 V above the nominal 1.8 V of the ADC VDDA causes some DC current to flow from the IO pins to the VDDA supply and might affect the ADC accuracy and specifications depending on the extra overhead voltage. Also it might cause long term reliability of the AM335x part. So as long as the voltage of the IO is under 2.1 V, and the current flowing through the IO pin is under 25 mA, there is no issue.

  • 0 in reply to Maher Sarraj
    Prodigy 30 points
    "As long as the voltage of the IO is under 2.1 V, and the current flowing through the IO pin is under 25 mA, there is no issue." -> Is this still true if the ADC supply rail is not powered?
  • 0 in reply to Jaap Overtoom
    TI__Prodigy 40 points

    Yes, provided the board setup or IO driver has a means to limit the current to under 25 mA. If the ADC rail is not powered and IO pin is at 2.1V, you potentially are shorting the IO pin to the ADC supply (Which is not powered and at 0V) through the ESD diode and could have very high current. We recommend to tie all the ADC IO pins to GND if the ADC is not powered.