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AD9851 output problem on power up with x6 refclk enabled

Question asked by Spacerowa on Feb 5, 2016
Latest reply on Feb 9, 2016 by Spacerowa

In my application there is no output from AD9851 when power up is enabled from a power down state if the x6 refclk multiplier is enabled.

Toggling AD9851 output functions normally, however, if x6 refclk is disabled.

The images below show what is being sent on the spi using 40bit serial word load. Byte 5 ( x6 refclk, power control, phase bits ) is shown; bits sent starting from left and ending on right. From the left, 1st bit is x6 refclk set to enable and 3rd bit is power control bit, set = power down. Just for additional clarification, ( but not relevant to the problem ), 8th bit is shown set for 180 degree phase change. Because of the image zoom setting the FQ_UD pulse is not shown off to the right, and frequency word bytes ( bytes 1-4 ) are off to the left; but frequency of output is always correct anyway, with or without x6 refclk.

 

screenshot1a.png

Above : x6 refclk enabled, power up enabled, 180 phase, hex 0x81

screenshot2a.png

Above : x6 refclk enabled, power down enabled, 180 phase, hex 0xA1

 

So why is there nothing on AD9851 output after going from lower image to top image ?

Anyone have any suggestions please ?

br, Spacerowa.

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