AD9913 amplitude is changing over the frequency. How to solve?

The output Amplitude of my AD9913 changes over frequency (for the same DAC code the amplitude is not the same).

The Ref_Clk is a 20MHz TCXO and is multiplied by 12 -->System Clock =240MHz.

The level seems very small, is that still correct?

Why is the amplitude changing? And what can i do against it? Is there a function behind it, so that I can calibrate it myself?

f in MHz dBm
30 -28,9
40 -28,2
50 -27,6
60 -27,2
70 -26
80 -25
90 -24,3

 all measured with DAC Code:1023 (which corresponds to the highest output current)