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ADS7142: ADS7142 Adress selection

worldwidewire
Prodigy 10 points
Part Number: ADS7142

Tool/software:

Hi we are using the ADS7142 ADC for a new PCB.

We have just ordered some prototype boards and noticed that on the adress selector we have connected the voltage divider to DVDD instead of AVDD.

The DVDD is 3.3V and also the reference is 3.3V.

Is there a risk that the adress is not selected properly?

The resistors have 1% tolerance.

And DVDD is generated by TLV74133PDBVR.

So the DC accuary of the LDO should be smaller than +-1%.

Thanks in advance.

  • 0
    TI__Genius 15510 points

    Hi Ireno,

    If AVDD and DVDD are the same voltage, there should not be a risk of an improperly configured address so long as the supplies are supplied at the same time, since the device configures its address on power up. Is there any delay between when AVDD and DVDD are supplied? If they are ramped at different times, say AVDD comes up before DVDD, then there is a risk of the voltage at the ADDR pin not being correct, and an incorrect address being configured.

    Regards,
    Joel

  • 0 in reply to Joel Meraz
    Prodigy 10 points

    Hi Joel,

    Thanks for your replay.

    As reference the REF35330QDBV is used.

    The NR-pin is floating, so there is no additional delay.

    In the datasheet of TLV74133PDBVR a startup time of 100us with 1uF load is mentioned. We have 2uF of output capacitance. 

    Both powersupplies have the same input voltage. 

    I think DVDD will ramp up a bit faster. 

    But i will do a measurement.

    How much delay is permissible?

    Can you provide more information about the addressing? At what time is the address set? 

  • 0 in reply to worldwidewire
    TI__Genius 15510 points

    Hello,

    AVDD is used as the reference voltage for the device. This device configures its I2C address by the ADC measuring the voltage at the ADDR pin, which is set by the resistors at the pin. Each address corresponds to a certain ADC code range. The address is configured when AVDD surpasses a certain threshold.

    Given the above, if AVDD and DVDD are supplied or operated independently, my concern is that if there is any difference in the time the supplies ramp up, or any voltage difference, the I2C address can potentially be misconfigured. If this design is produced at scale, variances in the power-up time, supply voltage, and residual decoupling capacitor voltage may contribute to that issue. I strongly recommend fixing this in a revision to reliably set the I2C address, else the workaround requires much more consideration and reliance on close supply timing.

    Regards,
    Joel