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ADS124S08: Input protection against miswiring

Shota Mago
Expert 8110 points
Part Number: ADS124S08
Other Parts Discussed in Thread: ADS124S06,

Hi team,

 

My customer considers to use ADS124S08 and provided a question.

In his end product, there is possibility that 24V is applied at AINx terminal by miswiring. In order to protect the device from this situation, does external resistor and internal diode(page31 in the D.S) at input enable to realize enough protection function?

 

If it's yes, how should we decide resistor value which keeps input within device rating?

Input Voltage: MIN：AVSS – 0.3 V MAX：AVDD + 0.3 V

Input Current: MIN：-10mA MAX：10mA

 

I'm looking forward to hearing back from you.

 

 Best regards,

Shota Mago

  • 0
    TI__Guru** 113765 points

    Hi Mago-san,

    The ADS124S08EVM shows an example of what the input would look like and the schematic can be found in the EVM user's guide.  You have specifically mentioned the criteria that must be met with respect to the Absolute Maximum Ratings for the ADS124S06.  The EVM uses 4.12k Ohm input resistors used as both the input filtering and as a current limit.

    The calculation for resistor sizing will depend on the AVDD supply voltage.  For example, if a unipolar supply voltage is used at 3.3V, the ESD diode will be fully conducting at 3.3V +0.3V or 3.6V.  If 24V is applied to the input, then the input resistor must be large enough to limit the current to 10mA or less.  This means that the resistor must drop the voltage 24V-3.6V or 20.4V with a maximum current of 10mA. 20.4V/10mA is 2.04k, so the resistance must be at least 2.04k and could be greater. For this situation, the 4.12k Ohms used with the EVM would be appropriate.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Expert 8110 points

    Hi Bob-san

    Thank you for your quick response and clear answer!

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Expert 8110 points

    Hi Bob-san

    My customer provided additional questions as follows.

    1. If input current is limited less than 10mA, protection diode doesn't never exceed 0.3V?
      If it' exceed 0.3V, it should be out of range of rating and the customer would need external diodes...
    2. Datasheet p31 says that  "overdriving an unselected input on the device can affect conversions taking place on other input pins."
      Does this problem occur even if input current is limited within 10mA?

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Guru** 113765 points

    As this question was posted in more than one location see the response here:

    https://e2e.ti.com/support/data-converters/f/73/p/895547/3311528#3311528