AD7859 Full Self-Calibration

I am having an issue getting the AD7859 to consistently perform a full self-calibration.

AVDD and DVDD are at 5V with a stable external 5V reference.

My clock is running at about 3.75MHz.

I do not see any activity on the CS.

About 1 out of every 25 times, when I pull the /CAL pin low for 10us and then release it back to 5V I do not see the BUSY line go high.

Because of this when my software goes to perform its first conversion, the chip does a calibration and my conversion fails.

Is there any documentation of using the /CAL pin to perform the full self-calibration as the datasheet only full explains using the /CONVST pin?

What are the potential reasons the full self-calibration did not start when the /CAL pin was taken from low to high?

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  • Jonathan,

    No problem.  I have checked the external reference and it is stable for at least 100ms before trying the calibration.  I have not tried the internal reference, only because most of my A/D inputs are higher than the internal reference.  I will look into it, to see if that may damage the part.

    I am writing to the test register to reset it back to its default value, although currently I am holding /CAL low when I do this.  The datasheet is not clear on whether that write will work or not, since the pin description for /CAL states that a low resets all logic and that the /CAL pin overrides all other internal operations.

    There is a snapshot of the schematic attached.

    Thanks,

    Mike

    AD7859.pdf
Reply
  • Jonathan,

    No problem.  I have checked the external reference and it is stable for at least 100ms before trying the calibration.  I have not tried the internal reference, only because most of my A/D inputs are higher than the internal reference.  I will look into it, to see if that may damage the part.

    I am writing to the test register to reset it back to its default value, although currently I am holding /CAL low when I do this.  The datasheet is not clear on whether that write will work or not, since the pin description for /CAL states that a low resets all logic and that the /CAL pin overrides all other internal operations.

    There is a snapshot of the schematic attached.

    Thanks,

    Mike

    AD7859.pdf
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