AD7795/96 Internal Offset Cal

I am using the AD7795 in a multichannel temperature application.  The product is calibrated at the factory with a system offset and gain cal before test. The calibration values are read from the AD7795 and stored in eeprom. They are loaded on power up. The current mode of operation is x16 gain, internal reference, chop enabled. I need to speed up conversions and turned off the chop and changed update rates to achieve this. I now see offsets.

My question is as follows:-

If I understand correctly, the internal offset cal removes errors in the ADC and the system cal removes errors external to the ADC.

If I add a periodic internal offset cal to remove the offsets caused by turning off the chop, won't I lose the system calibration zero offset ? Is there some way of combining the two ?

  • 0
    •  Analog Employees 
    on Jul 22, 2015 10:49 PM

    Hi analogpat,

         As mentioned on the datasheet, when chop disabled, the offset (internal or system offset) should be performed before the system full-scale calibration is initiated.

    Regards,

    Jonathan

  • Hi Jonathan

    yes you are right, I know the offset cal must be done before the fullscale cal.

    My question is regarding using a one time system cal, then employing periodic internal offset cals to remove the offsets and offset drift caused by switching off the chop. The internal offset cals will overwrite the system offset cal causing it to be lost. How can I combine them ? Add them together in firmware ?

  • 0
    •  Analog Employees 
    on Jul 29, 2015 11:18 PM

    Hi analogpat,

         I can suggest that first, it needs to perform a system calibration to get the external offsets and then do an internal offset calibration. For example,  perform first external offset, like have a zero input and then read the ADC result. Then this ADC result will then be use to correct all further conversion from the system. After then it will perform a periodic internal calibration without affecting the system calibration result.

    Regards,

    Jonathan

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:07 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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    EZ Admin