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Unstable output in Phase/Frequency detector AD9901


I am planing to build an all digital PLL using AD9901 as a phase detector. As far as I am informed, AD9901 has an internal phase/frequency detector to compute the error signal. When two square waves with the same frequency are applied to the two input (REF_IN and VCO_IN),the output swing is approximately +3.2V to +5.0V ,it really did a good job so far.However,question comes up when I change phase difference of the two inputs,the output suddenly became +3.2V dc,or sometimes became +5.0V dc, and sometimes it came back to right swint +3.2V to +5.0V.How strange it was since the inputs are right all along,the only diffrerence is the phase,but the output is unstable.

Another condition is,two inputs remain unchanged , when I touch the REF_IN or VCO_IN with a metal bar,the output will change between the 3 states mentioned before.

Does it mean that the output is unstable or it is sentitive to be affected by the other factors. maybe there are some more hints I should take into account. Thank you.

The following figure is my schematic diagram.



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