question about reference buffer with decoupling cap

[lotoy]

Dear all,
I have some problem on high speed buffer design, in which a big cap is used to get a low impedance at high frequency. what bother me is that there is a drift in transient simulation from DC value of reference output. I try to minimize the output impedance of opamp to reduce the drift,but there is still some difference between dynamic reference value and DC one.
pls help me out and give me suggestion!

thx in advance!

 

[safwatonline]

i think this is related to the buffer design i think it may contain systematic offset.