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AD7322: Accuracy and precision problem


I am currently trying to use a AD7322 but I have some problems with the accuracy and precision.

For exemple, I setup the input voltage at 3.76V (According my multimeter) and the range to ±10V as I use the internal reference (2.44mV is use to scale the data).

Bipolar mode:

Fully diff: 6.314 / 6.256 / 6.353 / 6.273 / 6.2268 / 6.322

Pseudo diff: 2.796 / 2.681 / 2.527 / 2.544 / 2.427 / 2.615

Unipolar mode:

Single Ended: 4.887 / 4.850 / 5.004 / 5.124 / 4.577 / 4.567 / 4.672

I attached the SPI command, it is the beginning of the configuration of the control register (0b10000010). The strange thing is that the input data is clocked on the fallaing edge of the SCK which is good but not the output data from the AD7322.

Please find also the schematic of the board.

I also have these components on another board where I use an external 3V reference but they also have this problems.

Any ideas?


Forgot to precise for the scope image

Channel 1 (Blue): SCK

Channel 3 (Pink): MOSI

Channel 4 (Green): MISO

Channel 2 (Cyan): CS
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