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AD5304+PIN work abnormal


I have test ADL5304 with PIN(PD), My PD is about 100nA  in the room.  I cover it up, PD is about  1nA.

ADL5304 is normal at PD 100nA, but it is normal at 1nA, Please give me some idea. 

Follow is normal picture at 100nA , it is about 1.5V.

Follow is abnormal pitcture at 1nA.

Follow is ADL5304 Circuit diagram。