A customer is using our AD9910 evaluation board, he uses an external signal source to supply a REF CLK. There is output at the DDS's output. But when he disabled the signal source's output, there is still an output at the DDS's output. I used the AD9910EVB to do the test. Agilent's 33250A is used to output 25MHz signal to AD9910 REF CLK input. After disabling the 3250A's output or power off to 33250A, there is a signal still a DDS output. So, I would appreciated if you can give some advice.