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AD9520-1 automatic reference switchover


I'm using the AD9520-1. I want to use the automatic reference switchover.

REF1 pin = 10MHz system clock.  DC-Coupled

REF2 pin = 10MHz local oscillator. DC-Coupled

When I fix the AD9520 to the REF1 pin, it runs fine. When I fix the AD9520 to the REF2 pin it runs fine. Now when I enable the 'automatic reference switchover' function, the AD9520 runs default on the local oscillator. When the system clock is there, it switches over to the REF1 pin.

This works fine for about two minutes, then the AD9520 switches back to the REF2 pin, go's out of lock for 1us and then switches back to the REF1 pin. This occurs every +/- 2min to +/- 5 min.

What is wrong? I would expect the quality of the REF1 or REF2 signal, but then again it runs fine if I force it to one of the REFs pins. Even though the signal looks very stable and good on the oscilloscope.

What is wrong??

Kind regards,