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ADS127L21: ADS127L21: Minimum Logic-Low Output Voltage for (SDO/DRDY and DRDY)

OMER SARI
Prodigy 20 points
Part Number: ADS127L21

Tool/software:

Hello,

I have a few questions about the ADS127L21. 

According to the figure shown above:

I am concerned about the maximum logic-low output voltage (VOL) for SDO/DRDY. I read that the maximum logic-low output voltage is 0.2 x IOVDD. My IOVDD is 5V, so VOL (maximum logic-low output voltage) would be 1V. Is this correct?

Or is this calculation specific for 1mA or 2mA current from the external pull-up resistor or current source?

For the SDO/DRDY internal RDS_on resistor calculation:

0.2 x 5V = 1V, meaning VOL_max = 1V at 2mA or 1mA, right? I will take 1mA current soure for this calculation.

The SDO/DRDY internal resistor would then be: 1V / 1mA = 1 kOhm correct?

According to the figure shown above:

I used a digital isolator, which has a pull-up current of 10µA at 5V (5V / 10µA = 500kΩ pull-up resistor), and the maximum logic low-level voltage is 0.8V.

When the ADC drives the output to a low state, the output voltage becomes (1kΩ / (500kΩ + 1kΩ)) x 5V = 0.01V. So, the output voltage is 0.01V, correct? Can we do the same calculations for the output as well?

 

I would appreciate a quick reply.

Thank you

  • 0
    TI__Mastermind 41901 points

    Hi OMER SARI,

    We did not get a chance to look into this question this week, we will provide a response early next week. Thanks for your patience

    -Bryan

  • +1
    TI__Mastermind 40950 points

    Hello Omer,

    Yes, the source resistance for the digital outputs can be modeled as a resistance.  The voltage drop verses current is not entirely linear, but it can be approximated.  In addition, the specification in the datasheet is worst case conditions, including minimum supply voltage of 1.8V.  Using an IOVDD voltage of 5V, the internal resistance will be lower.

    Your above calculations are a valid approximation.  Although we do not have specifications under your operating conditions, you should not see anywhere near 1V with a load current of 10uA (pull-up or pull-down) and IOVDD=5V.

    Regards,
    Keith Nicholas
    Precision ADC Applications