Finally I connected the ADC and a 1 MHz external sinusoid to its input (1 Vpp).

After some struggling and debugging I understood that the AD9226 board wasn't coding "two's complement" but as an unsigned integer i.e.

1  V -> FFFF  FFFF  FFFF

0 V -> 7FFF FFFF  FFFF 

-1 V -> 0000 0000 0000

Later I noticed (reading the AD9226 datasheet ... I didn't do it before) that AD9226 features:

Straight Binary or Two’s Complement Output Data

In fact the ADD9226 board has R35 and R35 to set Straight Binary (Factory Default) or Two's Complement.

As I preferred Two's Complement, I changed R34 and R35 in this way.

 •R35: not connected

• R34: 10KOhm