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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  • 0
    •  Analog Employees 
    on Apr 13, 2014 8:07 PM

    Hi Mike,

    Apologies for the delay.

    Now I understand that you are using the /CAL pin for self calibration. How do you do the Power up sequence of the AD7859 and the external reference? At power up the AD7859, the first /CONVST after AVDD and DVDD is on, the part sets 32ms (4MHZ CLKIN). This allows internal reference to settle before calibration is performed. You mentioned that you are using an external reference, please ensure that this must be stabilize. Kindly share the schematic, so we may have better understanding of the issue.

    Have you tried using the internal reference? just to check if the same result occur.

    On the p.25 of the datasheet also, it was mentioned that the Test Registers might be corrupted when the AD7859 is not operating as expected, a 16-bit word 0x0002 should be written on the AD7859 to restore Test Register content.

    Regards,

    Jonathan

Reply
  • 0
    •  Analog Employees 
    on Apr 13, 2014 8:07 PM

    Hi Mike,

    Apologies for the delay.

    Now I understand that you are using the /CAL pin for self calibration. How do you do the Power up sequence of the AD7859 and the external reference? At power up the AD7859, the first /CONVST after AVDD and DVDD is on, the part sets 32ms (4MHZ CLKIN). This allows internal reference to settle before calibration is performed. You mentioned that you are using an external reference, please ensure that this must be stabilize. Kindly share the schematic, so we may have better understanding of the issue.

    Have you tried using the internal reference? just to check if the same result occur.

    On the p.25 of the datasheet also, it was mentioned that the Test Registers might be corrupted when the AD7859 is not operating as expected, a 16-bit word 0x0002 should be written on the AD7859 to restore Test Register content.

    Regards,

    Jonathan

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