Output amplitute from an NCO

I have checked the clocks and can confirm that it is the same clock for both of them.

I will assume now your DAC has no sinc correction. You will get amplitude droop(due to hold device similar to low pass average filter for the duration of one sample). This droop goes towards zero at Fs and its multiples(after successive lobes). The remedy is to add inverse since filter somewhere e.g. in the fpga or in the dac(some dac have it) or in the analogue domain.

I have to say the output does look rather like a sinc function :P I am using the DAC / ADC on the stratix II card.

sinc distortion is caused by the hold effect of DAC(it doesn't affect the DAC value conversion itself, i.e. the ladder of values will always follow the conversion rules) but the hold effect is very much like any averaging low pass filter(analogue-wise).

The NCO IP has a frequency independent amplitude. Have you an idea, how it should perform different? I don't.

Its a stratix II 180. 1MHz to 47MHz

here is a matlab example of coefficients for inverse sinc

%inverse sinc filter(31 taps)
n = 31; 
f = linspace(-.5,.5,1024);
A = 1./sinc(f);
h = fftshift(ifft(fftshift(A)));
h = h(513-15:513+15);  %truncate
h = h/sum(h); %normalise

I previously missed some of the contributions in the thread, so my first post was somewhat behind the discussion. If you have e.g. 100 MHz DAC clock, the sinx/x effect is rather strong of course, and everything should be clear now. If you use the SLP-50 antialias filter, there's an additional 1 dB amplitude drop at 47 MHz.

So this must be a common problem when using the DAC on the S180 then?

The basic commodity DACs are supposed to only do the digital to analogue conversion and are the cheapest.