Phase coherent problem with two AD9164

Hello,these days I met a phase coherent problem with two AD9164.

Both two AD9164 use the same DAC CLK, and I config them work in NCO only mode, the output signal can phase coherent.

Change the NCO FTW, and use TX_ENABLE load FTW,also the output signal delay is fixed.

1,When I reset these AD9164 use a external signal, and then config two AD9164 work in NCO only mode. The output signal delay is changed. 

2,Not reset these AD9164, only switch the DAC CLK, it also change the output signal delay.

How can I solve this phase coherent problem.

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