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EVAL-AD7983-PMDZ unexpected offset


I driving EVAL-AD7983-PMDZ by Unipolar, Single-Ended sinusoidal signal (Vpp=4V, Voffset=2 so Vmin=0, Vmax=4).

R2 and R4 were removed from the EVAL-AD7983-PMDZ PCB because they give the additional useless voltage offset:

I see that samples are incorrect. Even if V+ is connected to the Gnd I see the offset:

I tried to slow down (to 1MHz SPI clk) the SPI interface but it does not help. SPI looks as usual:

What could be a reason for this ADC behavior?