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Ad7699 unipolar mode working abnormally

Hi,

   I am using  ad7699 adc . I want to acquire data from IN0 channel in unipolar mode with respect to COM =gnd. I am doing it in the RAC ( read after convertion process. My SPI is working in 3MBPS speed

I have set CFG register = 0x3879 

My program proces

  • Set CNV low.
  • Write the CFG register and receive dummy data
  • Set CNV high
  • Have a tconv delay
  • Repeat the process two times and get dummy data then again Set CNV low
  • Write CFG register and receive real data
  • Set CNV high

I have also tried to set CFG register = ( 0x 3879<<2 i: e 0xe1e4)

As it is written in the datasheet the CFG is updated in first 14 cycle 

I need a guide where  I am going wrong in the process as I am getting abnormal data ( like garbage ) . May I know the right process 

Regards.