AD7730 Internal-zero-scale calibration

Hi, I am using the AD7730 and i have some problems with the Internal zero-scale calibration. I use the SPI interface to communicate between my uC and the AD converter.

I am able to wirte and read from the registers of the AD7730.

The converter is configured in bipolare mode, HIREF = 1( 5V ), Range : +-10mV, 16bit mode.

I am using two AD7730 parallel which are selected with two CHIP-Select.

 

After a calibration cycle the value of the corresponding Offset-Register changed by a very small value but there is still an offset(approx. 30150) to the expected value of 32768 when the input pins are shorted.

 

Has anyone any advices?

 

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  • Hi RC

    First I have to say thank you for the support.

    In my last message I forgot to mention that the pins were shorted on a common-mode voltage of +2.5V.

    But this was also not the problem.

    After spending a lot of time I finally got it.

    Befor a calibration I had to disable the CHOP-Mode of the AD converter.

    If I do the calibration as follows it works and I am getting the expected value(32768).

    1. Set the required SF value in the filter-register, and disable the CHOP-Mode.

    2. Set the required value in the DAC-register.

    3. Internal full-scale calibraton(+/- 80mV).

    4. Wait for the /RDY flag in the status-register.

    5. Internal zero-scale calibration(+/- 10mV).

    6. Wait for the /RDY flag in the status-register.

    The value I received is ~32720.

    This is realy colse to the expected value.

    Regards,

    Michael

  • why this value 32768  as correct value?, where have you seen it?

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