LTC6948's SDO Pin

Hello,

If the LTC6948 is not used in a multi-drop configuration, the datasheet recommends a high-value resistor of greater than 200 kOhm between the SDO and GND.

However, since Ioh = -19.5mA (max), I think that there is no problem if the resistance value is 200ΩOhm or more.

If you need more than 200kOhm, please tell me the reason.

Best Regards,

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  • +1
    •  Analog Employees 
    on Sep 14, 2020 1:57 PM 5 months ago

    Hi,

    When the SDO pin is tri-state we recommend a pull-down (or even a pull-up) resistor so that the output is at a known state and not floating.  The max size of the pull-up or pull-down resistor is limitted by the leakage current in tri-state mode.  

    From the other angle of what is the minimum value of the pull-up or pull down resistor (the question you are asking) when the SDO pin is outputting data...  One concern is the absolute maximum condition of the device that can cause damage to the driver.  From this standpoint 200 ohms is fine.  The other concern is how much noise/spurs due you want the SPI to generate.  The more current pulled out of the SDO pin (i.e. smaller R pull down) will create more ground and/or supply bounce and possible cause coupling spurs on your hardware when SPI is running.  Based on this coupling concern higher values of resistance are better as they limit the current out the SPI output.

    The value recommended in the 6948 datasheet was a compromise in that it chose the highest R that minimized the ground/supply bounce and took into account the tri-state leakage current.

    Personally, I use SPI pull ups or pull downs in the 10k to 200k range as a general practical rule thumb for any device.  However, in this case the 6948 datasheet provided a theoretical best case value.  But if you used a 10k, 50k, 100k or 200k, practically speaking you probably wouldn't ever know the difference.  

    Chris

  • Hi,

    Thank you for your reply. I will measure spurious to determine resistance.

    Best Regars,

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