We have a corner case issue with the AD9957. We are using several AD9957 and combining the outputs externally on the PCB.
We have seen that some specific QDUC amplitude scale factor settings (register 0x0E, bit <55:48>) is creating something that looks like a phase discontinuity on the output. In the attached screenshot you can see samples of the combined signal. The part marked in red circles is a training sequence were the two combined signals are in phase (same bit pattern). When changing the amplitude scale factor from 0x79 to 0x80 (right and left plot respectively) on one of the two signals, the amplitude of the combined signal is dramatically changed. This seems to us to be linked to the phase of the QDUC output. When changing from 0x80 to 0x81, the combined output is back again to the normal level (same as 0x79). We have not had time to investigate all amplitude scale factor settings, but there are at least 3-4 other settings that we know of. This behaviour is consistant on several devices, so it is not an issue linked to a single component.
Do Analog Devices know anything about this?
Knut Olav Skyttemyr