AD9851 output problem on power up with x6 refclk enabled

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.

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

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.

  • 0
    •  Analog Employees 
    on Feb 8, 2016 6:02 PM over 5 years ago

    Hi Stephen,

    Whenever the ref_clk multiplier are enabled, the frequency range of the the clock input changes. This happens because the device needs to compensate due to having a limit in terms of operating frequency which is 180 MHz.

    So if you are using a multiplier, you also need to use a lower frequency. This the only idea I could think so far.

    Best Regards


  • Hello Louijie,

    My clock is always the correct frequency; the max changing appropriately depending on whether or not x6refclk multiplier is enabled. Output frequency of AD9851 is always correct.

    I had almost given up. However your reply encouraged me to look again at how my code procedure was handling toggling power. I experimented with some changes to the code. Before, I had just been toggling power enable/disable on its own with x6refclk mult enabled and unchanged. I have now discovered that if  I disable x6refclk mult and disable power in the same spi transaction, then enable both together in the same spi transaction there is output at the correct frequency from the AD9851.

    So I have found a solution which works in my case, though I don't know if it would apply generally and be of help to anyone else.

    Thank you for your time and input.

    br Spacerowa..

  • Hi,

    I came across the same issue. I am using 3 AD9851's in an application. I need to power down 2 of the three AD9851 in certain situations, and then power them up later. Through experimentation, I found that the oscillator would not work correctly if the power down command was issued with the 6X multiplier bit set. It seems the data sheet is incomplete , as the data sheet infers that the AD9851 ignores the value in W32 (Figure 20. Serial Load Power-Down/Power-Up Sequence) in powering down or powering up the AD9851. If W32 is zero on Power Down, then the device does power down and later power up as expected.