some questions about high precesion constant current source

hi,everybody.
I have some questions about hight precesion constant current source.My
goal is 1uA whose error is blow 0.01%.

In my circuit I used a op-amp,p-mosfet and 5V voltage reference.
But the result is so bad,the error is almost 1%.

Could somebody give me some hints?
Thanks very much.

eehinjor

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