about DAC‘s SFDR

[yxk]

Hi guys:

i have designed a 12 bit current steering DAC, when i calculate the SFDR of the output waveform, the verity of the value is so much.
since the output wave is step by step, when i take 1 point at one step, the output SFDR is 70dB, but when i take 256 points at one step, the output SFDR is 50dB.

who can tell me the reson of the phenomenon, and which one is resonable.

thanks.

 

[BrianSu]

For ur reason, i think maybe the result 70db of SFDR at 1 point/ 1 step is correct. When u test with 256 points with one step, the output freq would be lower 256times as original 1 step/1 point. So that u should change this freq value to 256 times lower in yr fft cal program, then the result would be near 70db.