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 10:02 PM

    Hi Mike,

    Apologies for the delay.

    The AD7859 after power is applied to AVdd and DVdd and the /CONVST is applied, the part requires 32ms + 1/sample rate for the internal reference to settle and for the self calibration to be completed. As you mentioned, you are using external reference, please make sure that this is stabilize.  Have you tried using internal reference? just to check if the same problem will occur. Please also follow recommended decoupling capacitors for the AD7859. You may also want to share the schematic diagram so we may have better understanding of the issue.

    When the AD7658 is not operating as expected, it is possible that the Test Register may have been corrupted. A 16-bit word 0x0002 should be written to the AD7859 to restore Test Register contents to the default value.

    Regards,
    Jonathan

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  • 0
    •  Analog Employees 
    on Apr 13, 2014 10:02 PM

    Hi Mike,

    Apologies for the delay.

    The AD7859 after power is applied to AVdd and DVdd and the /CONVST is applied, the part requires 32ms + 1/sample rate for the internal reference to settle and for the self calibration to be completed. As you mentioned, you are using external reference, please make sure that this is stabilize.  Have you tried using internal reference? just to check if the same problem will occur. Please also follow recommended decoupling capacitors for the AD7859. You may also want to share the schematic diagram so we may have better understanding of the issue.

    When the AD7658 is not operating as expected, it is possible that the Test Register may have been corrupted. A 16-bit word 0x0002 should be written to the AD7859 to restore Test Register contents to the default value.

    Regards,
    Jonathan

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