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AD9951: output frequency accuracy depending on sysclock?

Question asked by pfm on Mar 9, 2011
Latest reply on Mar 9, 2011 by pfm

Hello,

 

I am using the AD9951. My observation with this IC is that the output frequency accuracy depends on the system clock of the DDS; or in other words, depends on the multiplication factor in the CFR2 register. When generating an output frequency of 30 MHz at different system clocks, I measured following values:

 

SysClk = 400 MHz  ->  measured = 28.12 MHz

SysClk = 380 MHz  ->  measured = 29.58 MHz

SysClk = 360 MHz  ->  measured = 30.00 MHz

SysClk = 340 MHz  ->  measured = 30.00 MHz

SysClk = 320 MHz  ->  measured = 30.00 MHz

 

If the system clock is below 380 MHz, the accuracy is very high. What are possible reasons that the accuracy cannot be reached over the entire specified frequency range?

 

Thanks in advance.

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