Not being able to control the amplitude of the AD9850/51 DDS chip output
waveform makes it difficult to generate decent signalling waveforms.
However, this is possible by changing the current into the Rset, pin 12,
which sets the full scale amplitude of the D/A output. On my DDS board this
current is set by an innocuous 3.9k resistor. Replacing this with a FET
acting as a variable resistor allows a DC level to control the amplitude for
non critical applications - such as bandwidth limiting a signal by envelope
shaping.
The voltage at this Rset pin is maintained at approximately 1.26V and
current is sourced through the external resistor. By suitable biassing,
this current can be varied, so directly and linearly altering the amplitude
of the output waveform. On the Analog Devices web site WWW.ANALOG.COM
there is a tecnical note on altering the amplitude by digital control via an
external D/A converter. Search for 'AD9851' then under related topics look
for technical data.
Andy G4JNT
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