AD9363, lower 4 bits are always zero

I am using AD9363. When using BIST and invoking signal generator I find lower 4 bits are always zero coming in to DSP from ADI. With 12 bit data sign extended to 16 bits the bottom 4 bits should be populated too. Is that observation of zeros in bottom 4 bits specific to BIST?



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[edited by: andrei_g at 10:43 AM (GMT -5) on 28 Nov 2018]