Application and Circuit Description
We are using the AD9914 (DDS) to generate a 900MHz frequency modulated signal. The reference to the AD9914 can be either a 2.4GHz or a 3.25GHz LVPECL signal. A differential 2:1 LVPECL multiplexer, SY58018U from Microchip, is used to switch between the two references. A block diagram of the above application circuit is attached.
The PCB layout is currently underway and the LVPECL differential signals from the multiplexer to the AD9914 are crossing over as shown in the layout block diagram.
Engineer Zone Question
In order to facilitate the layout can I swap the AD9914 reference clock inputs?
That is can I apply the “Complementary Reference Clock Input Signal” from the Mux to the “Non-Complementary Reference Clock Input” of the AD9914 and simultaneously apply the “Non-Complementary Reference Clock Input Signal” from the Mux to “Complementary Reference Clock Input” of the AD9914.
In general can the “Complementary and Non-Complementary” signals of a differential interface can be interchanged.
I am fairly certain that it can be done but want to confirm.
Please refer the attached documents for more details.
Block diagram – AD9914_LVPECL_Reference_Clock_Mux_Block_Diagram.pdf
The REFCLK input requires ac-coupling, so making an "inverted" connection should not be a problem. Of course, there will exist a 180-degree phase shift between your originating clock source and the AD9914…
Thanks for the reply.
The DDS will be reset when ever the reference clock is changed.
The REFCLK input requires ac-coupling, so making an "inverted" connection should not be a problem. Of course, there will exist a 180-degree phase shift between your originating clock source and the AD9914 input.
The one thing to consider, however, is that switching between reference clocks on the fly cannot be done seamlessly. You will need to reset the part after switching reference clocks.
I am using ADCLK905BCPZ as a REFCLK buffer for AD9914 DDS. In my PCB if i put both the ic in top plane the clock signals are crossing each other. So can i connect the CLK of the AD9914 with CLK# of the ADCLK905 and CLK# of the AD9914 with CLK of the ADCLK905??. Will it make any problem in the functionality ??