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ADS1278-EP: Power input with respect to start up sequence

Christopher Ramirez
Prodigy 30 points

Part Number: ADS1278-EP
Other Parts Discussed in Thread: ADS1278,

I am adding some resistors to adjust the RC time constant of the power inputs for the purpose of properly performing the startup sequence in Figure 81 of the datasheet.

AVDD I am adding a 100ohm resistor

DVDD has  a 10 ohm resistor

IOVDD has an equivalent 50ohms

My question is what kind of power can i expect to dissipate across these resistors?

I did not see any info in the datasheet that could help me to properly size the resistors.

Any help on this would be very much appreciated.

Thanks 

Chris

  • 0
    TI__Guru 51470 points

    Hello Chris,

    These resistor values are too high, and will not work with the ADS1278.  You can find the ADS1278 current consumption specs for the different operating modes in the datasheet specifications section.  If you did use resistors in the power path, then the largest value for AVDD should be no more than 1ohm.  The power dissipation rating of the resistor would need to account for the charging of the bypass capacitors, but a standard 100mW rated resistor should be able to handle this.

    If you want to meet the power-up sequence shown in Figure 81 of the datasheet, then the best way to do this is to use RC values on the enable pins of the regulators that supply the AVDD, DVDD, and IOVDD pins.  However, you do not need to meet this power-up sequence if you assert the /SYNC pin after the supplies have ramped up to within recommended voltage ranges and after data first appear, indicated by /DRDY transitioning from high to low.  For example, the evaluation board does not always meet this power-up sequence, but pulses the /SYNC pin for proper operation.

    One other recommendation; I see that you have two different ground symbols for analog and digital.  Make sure on your board layout that all ground pins on the ADS1278-EP are connected to the analog ground plane directly beneath the device.  If you are using split ground planes on your board, then make sure there is a low impedance connection between the planes near the ADS1278 where the digital lines cross.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • +1 in reply to Keith Nicholas
    Prodigy 30 points

    Thank you for the quick reply. This is very helpful. I will talk with my lead about this but I do have one more question.

    What is the timing I need to meet for the startup sequence to work properly should we decide to meet this requirement after all?

    Do I need 500us between each power rail as they power up or what what kind of time frame are we talking about?

  • 0 in reply to Christopher Ramirez
    TI__Guru 51470 points

    Hello Chris,

    The power-up sequence for ADS1278 is only required for proper operation of the internal power-up reset circuit.  As long as DVDD reaches 1V or greater before IOVDD starts to ramp, and IOVDD reaches 1V or greater before AVDD starts to ramp, then the internal power-up circuit will operate properly. The time between each power rail then depends on the ramp rate.  If it is 1V/500uS, then you would need to wait 500uS, but if it is much quicker, such as 1V/100uS, then you would only need 100uS between each power rail.

    Please note that even if you meet all power-up requirements, we still recommend asserting the /SYNC pin after all power supplies have reached the nominal levels and data first appears.  This ensures proper operation in cases where the supplies are shut-down and the bypass capacitors are not allowed enough time to fully discharge before power is reapplied.

    Regards,
    Keith