The second lesson I learned was about the unbalanced -> balanced amplifier AD8138 feeding the balanced AD9226 input. Its analog input schematic for the board I used is something like this:

AD9226 12-BIT 65 SMPS  BOARD - original schematic

Therefore the overall amplification between VINA-VINB and INPUT_50_Ohm is approximatively:

200//62

-------------------- x (200/200 + 1) = 0,38 i.e. -8,4 dB 

200 + 200//62

Well, connecting my End Fed Long Wire Antenna to the Input_50_Ohm I realised that the AD9226 samples (12 bits) were around 0x820 and 0x07E. So I decided to amplify a bit :)