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ADS7828: I2C 3.3V logic level compatibility with ADS7828 Vdd powered by 5V

Kevin McConnell
Prodigy 20 points
Part Number: ADS7828
Other Parts Discussed in Thread: TLA2528

I have a design I am working on that uses the ADS7828 running Vdd at 5V.  The I2C bus is pulled up to 3.3V.  It seems to mostly work, but I am occasionally seeing some unexpected values returned from the ADC via I2C.  I am concerned that the ADS7828 can't reliably detect a logical high (3.3V) on the I2C bus.  According to the datasheet the minimum logic high input threshold is (Vdd * 0.7) which in my case Vdd is 5V which would mean the minimum threshold is 3.5V.  

That said, how could the 3.3V I2C bus be working at all with this part powered by 5V?  Could it work, but with very little margin?

  • 0
    TI__Mastermind 24940 points

    Hello,

    The device is likely to still function, but just barely. Like you were saying, you are right at the threshold, which means that any fluctuation could make the device function or not. As for how much margin there is, that cannot be said, the parameters given in the datasheet are the only numbers that we can go by.

    That being said, this is not a reliable use of the device.

    I would suggest looking at the TLA2528, this is also a 12 bit, 8 channel device. But it has two separate power supplies for the analog and digital rails. This means you can have a 5V full input scale but correctly and reliably use a 3.3V bus.

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Prodigy 20 points

    Hi Cynthia,

    This is as I thought then.  I suppose I can either adjust my analog voltage rail down lower or look into the TLA2528.  Thank you for the suggestion.

    Regards,

    Kevin