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ADS7038-Q1: AINx is supplied and AVDD/DVDD not available

Fabian Range
Prodigy 20 points
Part Number: ADS7038-Q1
Other Parts Discussed in Thread: TPS745-Q1

Hi Team,

we want to measure in our application the battery voltage of 32V. It is connected with an voltage divider, to that we limit the current at the input pin to max. 0.57mA.

But for sure the voltage will be available (due to power sequencing) before AVDD and DVDD is available. Is this a problem?

With a look on the absolute maximum ratings we would exceed the maximum allowed voltage, since it is dependent on AVDD. Can we destroy the ADC during power up phase in this scenario?

Thank you,

Fabian

  • +1
    TI__Mastermind 24940 points

    Hello,

    Your observation is correct that this is not ideal and would violate the abs max

    The device though, with current limiting should be fine. The analog input pins have ESD diodes that will turn on when the analog input exceeds the power supply this will then redirect the current to the AVDD rail. This is important to note, because you power rail will no longer be at zero during this time. 

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Prodigy 20 points

    Hello Cynthia,

    thanks for your quick answer. Maybe one additionally question. Do you have any recommendation how to measure voltages which are supplied before AVDD rail is available? We currently thinking about adding FETs to input lines to switch the input voltages during start-up off. But of course that are more components, we'd like to reduce.

    We are currently unsure if the leakage current into the AVDD rail does harm our LDO regulator. We are using the TPS745-Q1 from TI. It seems that is has an internal discharge circuit with 95 Ohms which would fit for our leakage current of 2.2mA (in total of 4 rails). But most probably this discharge circuit will not be active when also the LDO is unpowered.. So this might not solve our problem.

    Thanks,

    Fabian

  • 0 in reply to Fabian Range
    TI__Mastermind 24940 points

    Fabian, 

    The device will not be able to operate and complete a conversion to measure the analog input without AVDD rail available. 

    To protect the device during the situation where AVDD<AIN, adding a series resistor to the analog inputs can limit the current. 

    TI has a video series on several topics, one being electrical overstress on data converters, and within this topic, I would suggest watching or reading through topics 11.5 Protecting low voltage: improved solution ~ click here ~ and topic 11.7 Protecting ADC with TVS diode: Improved solution ~click here~

    Regards

    Cynthia