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Why after every conversion of AD7091R is the MSB of the conversion result always “0”?

If the communication timing is ok this is usually due to the time between the /CONVST start pulse going low and reading back the conversion. The conversion time is 650ns maximum. If the  result is read back before this time has elapsed then it is possible that the MSB will be zero.

You can also use the BUSY indication feature to indicate when a conversion is complete by having /CS low prior to the EOC point. This will take the SDO line out of high impedance once a conversion completes and this can be detected by the SPI master.