AD9166 EVB output is nonlinearity more than dozens of LSBs

I test the DC output of AD9166 (AD9166-FMC-EBZ). 

The configure of ACE is as follows:

Use DPG Downloader to send DC value,everytime I change the Value. I use Tek 6Series oscilloscope at 50Ω load and 20MHz Bandwidth.Averge mode(2048 times),and the results are mean value of oscilloscope. Oscilloscope is working at 2mV/div.

The test results:

value results/mV
255 55.876
256 55.837
257 55.692
258 55.425
511 50.286
512 50.176
513 50.201
767 44.746
768 44.699
769 44.552
1023 38.617
1024 38.473
1025 38.301
1026 38.191
1027 38.177
1028 38.141
1029 38.058
1030 37.951

By using matlab the plot is :

It can be seen that there are large step on 0x100, 0x200, 0x300, 0x400. Why? I guess that in other value such as 0x1000, 0x1100, et . al will also exist large step.

Can anyone give me some help?