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AD5932 Strange Output Frequencies

Question asked by jake.m on Oct 29, 2010
Latest reply on Nov 5, 2010 by jake.m

I am using the AD5932 with a 50MHz clock. I'm unable to get the proper output frequencies.

 

The control register and the number of increments register both work properly, but I get the same strange frequency behavior regardless of how I program the frequency registers.

 

The output frequency increments as follows:

 

5kHz,  ~18MHz, 12.5MHz, 6.25Mhz, 25MHz, 6.25MHz, 12.5MHz, ~18MHz, ...

1.9kHz, ~18MHz, 12.5MHz, 6.25Mhz, 25MHz, 6.25MHz, 12.5MHz, ~18MHz, ...

1.2kHz, ~18MHz, 12.5MHz, 6.25Mhz, 25MHz, 6.25MHz, 12.5MHz, ~18MHz, ...

4.3kHz, ~18MHz, 12.5MHz, 6.25Mhz, 25MHz, 6.25MHz, 12.5MHz, ~18MHz, ...

7.3kHz, ~18MHz, 12.5MHz, 6.25Mhz, 25MHz, 6.25MHz, 12.5MHz, ~18MHz, ...

10.4kHz, ~18MHz, 12.5MHz, 6.25Mhz, 25MHz, 6.25MHz, 12.5MHz, ~18MHz, ...

13.5kHz, ~18MHz, 12.5MHz, 6.25Mhz, 25MHz, 6.25MHz, 12.5MHz, ~18MHz, ...

 

This pattern continues with one frequency in the kHz range followed by the same seven higher frequencies. The kHz frequency continues to increment by 3kHz. I get these same frequencies no matter what values I write to the start frequency and increment frequency registers. I have confirmed on a scope that the correct data is being sent to the DDS.

 

Here's an example of values I have written to the registers (Fstart = 100kHz, df = 50kHz)

 

Frame 1: 0x0FFF //Control register - I am able to see the correct effects of changing these bits

Frame 2: 0xD008 //Fstart MSBs

Frame 3: 0xC312 //Fstart LSBs

Frame 4: 0x3004 //df MSBs

Frame 5: 0x2189 //df LSBs

Frame 6: 0x10A0 //Number of increments - I am able to see the number of increments change correctly by changing this value

 

Given that the first and last frame work properly, it seems that the data is being written correctly. Do you have any idea what could be causing this issue? I'll be able to test on another board soon to see if there is a problem with this chip.

 

Thanks for the help,

Jake

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