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ADS7844: Clarification of Absolute Maximum Input Voltage Rating and Default Control Register State

Prodigy 60 points

Replies: 2

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Part Number: ADS7844

Hi,

In the datasheet for the ADS7844, the absolute maximum input voltage rating for the analog inputs is given as -0.3 V to VCC + 0.3 V. However, in the specification table, an "absolute input range" for the negative inputs (when operated in differential mode) is given as -0.2 V to 1.25 V. I believe that "absolute" has been used in this context in order to differentiate from the differential input range rating, and that this should not be interpreted as an absolute maximum rating. (E.g. if a negative input is driven to, say, 1.5 V, the conversion will not be accurate, but no damage will occur.) Is this correct?

Further, the datasheet does not provide default values for the control byte bits (Table III), so it is unclear whether the converter will start up in single ended or differential mode. What is the default value of this register?

Thanks

Shane

  • Hello,

    Your understanding is correct about the ratings for the pins. Each analog input can sustain a voltage up to VCC, but when in differential mode, the voltage at IN- should be within -0.2V to 1.25V, for bet performance. Outside of this, as you noted, the device will not be damaged but will not perform as expected. 

    The start up logic of the SGL/DIF does not matter, as the control byte needs to be set before a conversion takes place. the device will not start a conversion without the control byte, thus the default logic of this register is never actuated. 

    Regards

    Cynthia

  • In reply to Cynthia:

    Thanks for the prompt and clear answer Cynthia.