In table 8 of AD9915 datasheet, recommended values of charge pump currents of some N ranges is indicated but for values of N greater than 100, there is not any recommended value. What is the charge pump recommended value for N greater than 100?
The charge pump current is controlled by using 3 bits data. At N divider with value of up to 100, the recommended current is 1000 uA which is already the full scale current (CFR3[5:3] = 111) of the charge pump. I haven't tried it yet but since it could not go beyond 1000 uA, I recommend to use that value instead.
So the greater values of N are useless?! According to the datasheet (Page 21, PLL multiplier) 2N could be between 20 and 510.
Not that it's useless but it is rare to use those values. After all, the VCO frequency range is at 2.4 to 2.5 GHz. The crystals that are typically used are within 10 to 30 MHz which requires a multiplier of N = 120 (or 2N = 240). Well it depends on the application, In your case, will you be needing a very high multiplication factor?
I want to use a precision 10MHz crystal oscillator and N=122. So fsys would be 2.44GHz. Actually I want to make two signals of 100MHz and 110MHz with DDS. I don't want fsys counts neither 100MHz signal nor 110MHz signal.
SInce you are using a high value multiplier. I suggest to use the maximum charge pump current value considering that you will reference your loop filter design to the one that we have in the evaluation board.
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