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ADS1119: SDA and SCL Input state with no power

FXtreme Electronics
Prodigy 210 points
Part Number: ADS1119
Other Parts Discussed in Thread: TCA9416

Hi:)

When the ADS1119 has no power, is it still possible to have a functioning I2C bus connected? I2C bus operates at 3.3V.

Thanks!

  • +1
    TI__Guru** 113765 points

    Hi FXtreme Electronics,

    The short answer is the ADS1119 was not designed or tested to verify if operation will continue on the I2C bus with the ADS1119 unpowered.

    It may be possible under certain conditions, but the design was based on a powered device.  At the time of design development there was no request to place the device unpowered on the I2C bus.  If you look at the datasheet, all specifications are based on the device being powered.  To save power, the use of the POWERDOWN command can be used.

    There are a number of considerations.  As the SCL input is strictly an input, this pin proposes no issue.  However the SDA line can be both and input and an output and this is where there could potentially be an issue if SDA is pulled or held low.  So the design of the I/O cell has to accommodate the power off condition.  Our next generation of I2C devices will take this into account.

    Another condition that can affect the ADS1119 is the device will be held in a RESET condition until all supplies (AVDD and DVDD) are made active and at nominal operating voltage.  Here it is unclear of your intent as to how you are powering the ADS1119.  Are you just turning off AVDD? Or are you turning off both AVDD and DVDD?  What some customers think is a good idea to conserve power, may actually consume more overall current than just keeping the supplies active and using POWERDOWN.  The actual value of turning off the supplies will depend on how long the supplies are in the off state as compared to the on time.  If charge on bypass and other associated caps have completely dissipated, then you have to consider the additional length of time for the device to recharge the caps and on time of the ADS1119 while the device is starting up.  So there are some tradeoffs to consider.

    Also, any powered sensors to the ADC inputs must also be powered off to prevent issues with Absolute Maximum Ratings violations at the inputs.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 210 points

    Hi Bob:)

    -

    Thanks for your detailed answer, it is really appreciated!

    -

    In my case the ADS1119 will be powered from a different supply than the main MCU. The supply for the ADS1119 will turn off when the system turns off, while the MCU its supply stays on indefinitely as long as a battery is connected.

    -

    What you said about the ADS1119 pulling low the SDA line was indeed my concern. So instead of connecting the I2C directly to the ADS1119, it might be a better solution to use a I2C switch like the TCA9416 in between since this will disconnect/isolate the I2C-Bus from the ADS1119 when either VCC input is low/GND.

    What are your thoughts on this approach?

    -

    Thanks!

  • 0 in reply to FXtreme Electronics
    TI__Guru** 113765 points

    Hi FXtreme Electronics,

    I have not used the TCA9416, but it would appear to be the appropriate device for the application you describe.

    Best regards,

    Bob B