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,

    Thank you for your reply.  However, I am not using the registers to initiate the calibration.  I am using the /CAL pin (pin number 16 on the AD7859AS).  From page 7 of the datasheet:  "Calibration Input.  A logic 0 in this pin resets all logic.  A rising edge on this pin initiates a calibration.  This input overrides all other internal operations."

    My clock is 3.75MHz, which should give a calibration time of about 33ms.

    My external reference is stable.

    I pull the /CAL pin low for 10us, and then set it back high.

    Most of the time I get a 33ms pulse on the BUSY (passing calibration).

    Sometimes I do not get a BUSY pulse at all, and sometimes I get a BUSY pulse of about 100ms (which indicates a failed calibration).

    I need to know what could cause a calibration to fail.

    And I need to know what would cause a calibration not to start from the /CAL pin.

    Thanks,

    Mike

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  • Jonathan,

    Thank you for your reply.  However, I am not using the registers to initiate the calibration.  I am using the /CAL pin (pin number 16 on the AD7859AS).  From page 7 of the datasheet:  "Calibration Input.  A logic 0 in this pin resets all logic.  A rising edge on this pin initiates a calibration.  This input overrides all other internal operations."

    My clock is 3.75MHz, which should give a calibration time of about 33ms.

    My external reference is stable.

    I pull the /CAL pin low for 10us, and then set it back high.

    Most of the time I get a 33ms pulse on the BUSY (passing calibration).

    Sometimes I do not get a BUSY pulse at all, and sometimes I get a BUSY pulse of about 100ms (which indicates a failed calibration).

    I need to know what could cause a calibration to fail.

    And I need to know what would cause a calibration not to start from the /CAL pin.

    Thanks,

    Mike

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