AD9913 Amplitude is chaning over the frequency

Hello :)

The output Amplitude of my AD9913 changes over frequency. I measured with an oscilloscope with an input resistance of 1MOhm. And i changed the DAC code, so i can alter the amplitude by myself. But 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.

Why is the amplitude changing? And what can i do against it?

f in MHz mV_pp
30 12,6
50 16
60 23
70 40
80 150
90 60

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