Hi Mr.K,
For a single supply configuration, Vs = +5V. The input bias current of the AD8641 at 25°C is 0.25pA(typical value) to 1pA(maximum value). However, if the ambient temperature is ranging between -40°C to 125°C the maximum input bias current of AD8641 can go up to 180 pA. Kindly refer to the table below.
Below is the input bias current range for a dual supply configuration, Vs=+/-13V.
In addition to that, the reason why the TEMP pin of the REF02 needs to be buffered with an op amp with a low input bias current is to minimize the current being drawn from the TEMP pin. It helps to prevent the voltage output of the REF02 to fall out of regulation.
Regards,
Jay