AD9786: Current output voltage calculation

Document created by analog-archivist Employee on Feb 23, 2016
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I found a problem with one of my customers where AD9786 full scale output power
under single tone test condition is only 3 dBm, much lower than the specified
value 10dBm, so they would like to confirm if max. output power of AD9786 is
able to achieve 10 dBm?


We don't actually say you can get 10dBm out of the DAC. We specify that the
full scale current is 20mA, and if you drive that into 50 ohms on each DAC
output, you will get 1V p-p on each DAC output. A 1V peak signal corresponds to
700mV RMS, which in turn equals 10dBm. That's probably where they are
calculating the 10dBm from. However, if you look at our characterization
circuits, for best performance, we recommend something lower. Much of our
characterization data is done with a single tone which gives 333mV p-p out of
each side of the DAC. This results in a 235mV RMS swing out of the DAC which
ends up being 0.46dB into the 50ohm input of a spectrum analyzer.  

I think it's most likely that they are not taking into account the 50 ohms of
their spectrum analyzer when they are doing the power calculation,