Interfacing current driven differential output to voltage driven differential input
 

Hi All,

I am trying to interface the output of an Analog Devices 9857 DDS modulator
to a Maxim upconverter. The problem is that the output of the DDS is a
current driven differential DAC, and the input to the upconverter is a
voltage driven differential input. No problem, you say? But there is
one...

The output of the DDS has spurs and aliases caused by the fact that this is
a signal generated by the DAC from digital data. SInce I would rather not
upconvert the spurs, etc. as well as the desired signal, I need to filter
the output of the DAC. I want to keep the differential signal for
noise-reduction purposes. The guy I spoke to at Maxim says that I must
filter both arms of the DAC output before feeding the signal to the
upconverter. Another alternative, of course, is to just connect the two
with a shunt resistor and filter the upconverter output, but I would rather
reduce the spurs as soon as possible rather than have them amplified on the
board. Is there an efficient, elegant way to do this? And yes, I have also
thought of taking the differential output, converting it to single ended,
filtering it and then converting it back to differential...that's why I
emphasize "elegant" above :)

Thanks in advance