how to raise input impedance of small signal

Hi,everybody.
The voltage of my signal is of mv level.this signal is the input of
ADCs.I want to raise the input impdedance of the signal,then the signal
will be isolated from the ADCs.

Simplely,An op-amp(OPA602) is used.

the signal---PIN3(IN+)
PIN2(IN-)--PIN5(OUT)

but I found the result is so bad.Such as,when the signal is about
2.5mV,the output of op-amp is almost 3.3mV.

Who know the reason and better ways?

Best regards.

.