quartus II NCO ip core to get constant voltage for different frequency generated

Hi all,

By using a "cyclone V" "5cgxfc5c6f27c7N" board and quartus II trying to generate sine wave using IP Core ,capable of generating different frequency with constant peak to peak voltage.

In quartus II i used NCO IP core to generate sine wave where i can change the frequency by giving input to "phi_inc_i" input pins ,i connected R2R ladder DAC to convert it to analog.

I get sine wave according to "phi_inc_i" value , but i am not able to get constant Peak to Peak voltage ,as i increase the output frequency Pk-Pk votage decreases.

please help me how to mentain constant pk-pk voltage for all different frequencies.