Case update and summary:
The rise in HD3 distortion level at low frequency spectrum (i.e. 1.25MHz) is due to the transformer bandwidth and also the spectrum analyzer input bandwidth limitation.
When the customer compare the result between their PCB board and the DAC3283 EVM, they noticed that the DAC3283EVM yields worse performance than their board. The DAC3283EVM default transformer is ADT4-1T, which covers the bandwidth of 9MHz to 625MHz. This transformer increases the HD3 distortion at low frequency below 9MHz due to the impedance variation of the transformer at this region. After changing the transformer to ADT4-6T with range of 60kHz to 300MHz, the HD3 distortion level improved to 90+dBc level.
When measuring the DAC distortion at low frequency range, the spectrum analyzer's input structure is important. The customer was able to overcome the measurement limitation on the spectrum analyzer side by setting their spectrum analyzer with DC coupling setting. Further adjustments of DC coupling caps were made to improve the measurement result.
TI lab confirms the result with R&S FSEB30 spectrum analyzer with default DC coupled analyzer input. The distortion performance of the analyzer itself is confirmed by injecting a high quality 1MHz source from the HP8644B signal generator with 1MHz BPF. HD2 and HD3 distortions at 2MHz and 3MHz are in the 90+dBc range.
TI will continue to work with the customer on this case offline to provide further support if needed.
-Kang