• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 534 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS131E08: are there any clamp diodes at the analog input pins?

Howard Zou
Genius 12365 points
Part Number: ADS131E08

Hi,

are there any clamp diodes inside the chip at the analog input pins?

My customer's analog input voltage may exceed AVDD, they add a current limit resistor at the analog input pins, do they still need external clamp diodes to AVDD?

  • 0
    TI__Guru* 97775 points

    Howard,



    Yes, the ADS131E08 does have clamp diodes at the analog inputs for ESD and overvoltage protection. This is common for many of our ADCs.

    If the customer's input voltage exceeds AVDD, the current limiting resistor may help. However, I would limit the input current to well below the Absolute Maximum Rating of 10mA.



    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Genius 12365 points

    Joseph,

    we have tested with a 1k current limit resistor in front of the ADC analog input.

    7.8V is applied to the current limit resistor, and we will measure 7.5V at the analog input pin, why? The current is only 0.3V/1k=0.3mA.

    The device can still sample the signal well when we decrease 7.8V to within Vref so the device is not broken.

  • 0 in reply to Howard Zou
    TI__Guru* 97775 points

    Howard,


    With your test, the input at the series resistor has been put all the way up to 7.8V. I'm not sure why the ADC analog input goes all the way to 7.5V. What is the AVDD voltage at that point? It could be that the AVDD supply is only capable of sourcing current, but not sinking current. If you apply a higher voltage to the analog input the AVDD supply may be pulled up. Regardless, the absolute maximum for the AVDD supply is 5.5V. I would be careful not to violate any of the absolute maximum ratings from the datasheet or there may be damage to the device.

    Do you have a schematic that you can share? It might help to analyze the circuit with a schematic of the device and the power supply.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 97775 points

    Howard,

    Just so that anyone reading this post knows, I've done some measurements with the ADS131E08EVM to see the input over-voltage behavior on the device. Based on the input voltage and current measurements that I've seen, the input (from the EVM) looks like this:

    This is why the input pin voltage rises so high. The 6kΩ input resistance inside the device can drop a significant voltage as the input voltage rises.

    As I mentioned in my direct email to you, I would limit the input to the ratings in the datasheet. I still think that 7.5V is too high for the input pin, even with the series impedance on the inside of the ADC. It may be useful to add some input protection diodes to VDD on the outside of the device to limit the input voltage.

    Joseph Wu