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Original question:

ADS1258-EP: ADS1258-EP

ADS1258-EP: Input Protection Unpowered Device

Chris Scioscia
Prodigy 10 points
Part Number: ADS1258-EP

Assuming a condition where the ADS1258-EP is unpowered on the digital & analog supply voltages: the Abs Max datasheet lists 10mA continuous (to protect ESD diodes).

1) Is that 10mA max the limit for each of the Analog & Digital Inputs? Or is it a 10mA max limit for all inputs on the whole package?

2) Page 42 has recommended input protection – however the leakage on the diodes can impact the analog signal & accuracy and for this reason they want to be avoided. Would just a limiting resistor be sufficient protection during an unpowered event?

3) 10mA is the abs max continuous limit - as stated in datasheet you would never want to operate this high - and obviously lower is better. Are there any suggestions at a reasonably safe current limit to meet on the inputs of this ADC during an unpowered event. Not looking for any sort of binding recommendation from TI, but a ballpark number that a designer could feel good about... if they had to design for an unpowered event with voltage on the inputs - any insights here are appreciated.

4) Page 25 - are there any sequencing requirements between the analog supply voltage and digital supply voltages? (Digital last on, first off?)

Thanks,

-Chris

  • 0
    TI__Mastermind 40141 points

    Hi Chris Scioscia,

    Here are the answers to your questions:

    Chris Scioscia said:
    1) Is that 10mA max the limit for each of the Analog & Digital Inputs? Or is it a 10mA max limit for all inputs on the whole package?

    The 10mA limit is per pin, which is the current limit on the ESD diode on each pin. But of course that current must come from somewhere, so if you have 10mA on each of the 16x input pins then that total 160mA will be drawn through the ground pin. This is not a sustainable situation, though I don't have any data to share with respect to how long the ADC could sustain such an event. It is best to design your system to limit the current to <<10mA.

    The ESD diode will turn on when the input voltage exceeds the supply voltage as described in the datasheet. Resistors in series with the input pins will limit the current into the ADC, so size them accordingly e.g. if the max input voltage is 5V then a 5k series resistance limits the input current to 1mA in the case that the supplies are not applied (AVDD = AVSS = 0V). Turning on the ESD diodes with unpowered supplies can potentially backpower the chip, so we do not recommend you allow this to happen, or you do so at your own risk. 

    Chris Scioscia said:
    2) Page 42 has recommended input protection – however the leakage on the diodes can impact the analog signal & accuracy and for this reason they want to be avoided. Would just a limiting resistor be sufficient protection during an unpowered event?

    The current limiting resistors will limit current of course, but will not protect the ADC in case of a transient event. TVS diodes are recommended if you expect these types of events to occur.

    Chris Scioscia said:
    Are there any suggestions at a reasonably safe current limit to meet on the inputs of this ADC during an unpowered event. Not looking for any sort of binding recommendation from TI, but a ballpark number that a designer could feel good about... if they had to design for an unpowered event with voltage on the inputs - any insights here are appreciated.

    See the answer to your first question

    Chris Scioscia said:
    4) Page 25 - are there any sequencing requirements between the analog supply voltage and digital supply voltages? (Digital last on, first off?)

    As the datasheet says, the supplies can be sequenced in any order

    -Bryan