Why do I see a very large signal at 2*fdac for the AD9739 DAC? It is not clear
to me why I see a very large 100mV amplitude signal at 2xfDAC. This corresponds
to 3mA switching current. Why is this so large?
Many of our DACs, including the AD9739, use a proprietary DAC switching
technique called the quad-switch. This removes code-dependent clock spurs and
creates a constant high-energy spur at 2*Fs. In the time-domain, the unfiltered
DAC output will have high-energy glitches due to this reason.
Since these glitches reside at 2*Fs, the analog filter following the DAC should
be able to attenuate them significantly. Note that on products such as the 9707
(which do not use this switching technique), the unfiltered DAC output will
look a lot "cleaner" in the time domain.