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AD9958 linear sweep problem

Question asked by jeffreyprinzie on Mar 21, 2013
Latest reply on Mar 22, 2013 by jeffreyprinzie



I am building a circuit with 2 channels.  Both at fixed frequency but CH1 has a linear phase shift 0-360° and is used as phase interpolator.


The interface to the DDS works. I can program both frequency correctly.


The problem comes when i try the linear sweep with NO-DWELL active.


The code initializes the CTFW0 frequency register to 12.5MHz for both cannels. (that works) .


Then an initialisation code is invoked that writes following registers:


Select channel 1



     0x0000 (S0)


     0xFFFFFFFF   (E0)

-RDW and FDW


-LSRR (40 ns per increment)




     AFP=11 (phase)




     rest is zero  (including open[12:11] bits)



In FR1 is set the profile pin configuration bits all 0, also RU/RD bits, because I dont use modulation.




In the datasheet it is noted tha P1 should be used as profile pin, but I tried all profile pins, no sweep is show, just steady sines at 12.5MHz having constant phase.


do you have any idea what can be wrong?



3.3V profile pins

25MHz Xtal



Thanks in advance