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ADS1115: Recommended current limit for ESD diodes forward-conducting from AIN to VDD?

Jonathan Bender
Expert 7895 points
Part Number: ADS1115

Hello,

On ADS1115, I have 12V connected through a resistor divider to AIN. The high-side resistor is 15 kohm. In some cases, the 12V signal may be present before the ADS1115's VDD, so we could have ~800 uA conducting through the AIN ESD protection until VDD comes up.

The datasheet notes that if this case can't be avoided, the AIN current should be kept below the absolute maximum, which is +/- 10 mA continuous for all non-power pins. Can you verify that this +/- 10 mA explicitly refers to the internal ESD diode's capability?

If so, it would seem that < 1 mA should be okay. I've seen internal ESD diodes on other devices that weren't capable of 10 mA continuous, so I wanted to double check that I understood this datasheet spec properly.

Thanks,

Jonathan

  • 0
    TI__Guru* 97515 points
    Jonathan,


    Generally, having an input current < 1mA should be ok. The 10mA max does refer to the shunt current for the ESD diode.

    However, I would not ignore the rest of the absolute max specifications. If you have a 12V supply, even limiting the current through 15kΩ, will VDD to GND stay below 7V? If the current is coming into one of the AINx pins, will AINx stay below VDD + 0.3V?

    Note that you would want the VDD supply to be operational so that the diode current shunts back to the VDD supply. I'm not sure what happens if VDD is 0V or if it is disconnected.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Expert 7895 points

    Hi Joseph,

    Thanks for the confirmation on the 10 mA rating applying to the ESD diode capability.

    I'm not entirely sure I follow your other questions, so let me share a quick diagram showing the circuit implementation. The top instance shows when 12V is present and VDD is not (assuming VDD = GND here). This would violate AIN < VDD + 0.3V, so the diode would be forward conducting, with < 800 uA.

    The bottom instance is in normal operation, with the typical 6 MEG input impedance for FSR +/- 4.096V.

    Is the top instance acceptable? I'm assuming yes based on the diode current being well below the 10 mA abs. max spec (plus the fact that the state will only be for < 1 sec, until VDD comes up), but I'm unsure of whether there are other factors that should be considered.

    Thanks,

    Jonathan

  • 0 in reply to Jonathan Bender
    TI__Guru* 97515 points

    Jonathan,

    I'd forgotten that you'd still have the 3.92kΩ resistor to keep AINx low. So you should be ok. I was worried that the 12V supply would pull VDD up to a much higher voltage.

    Note that without the VDD connection, your remaining ESD diode is a reverse bias connection to GND, not a forward bias to GND as is drawn in your schematic.

    Joseph Wu