AD8436 noise senstivity at startup

Use AD8436 on DMM project, encoutered trouble as following:

1) with 0mV input (AC input grounded with 100kOhm), after power up, AD8436 firstly got 0mV output, then had ~2mV output after 10-30s, the duration was not fixed, ie some times longer and sometimes shorter. after the duration, the output of AD8436 stick to ~2mV later

- See attached picture for the test setup with EVB.

- AC couple, IBUF enabble, OBUF enable, dual supply+/-3V, 100kOhm to ground.

2) Expect the output AD8436 can be kept at 0mV

- Compared with AD637, the inital output of AD637(0mV input) is typically x-mV level then getting down to 0mV, it is acceptable and almost no need for calibration.

- with AD8436, the initial output(0mV input) is 0mV, but will ramp up to a stable value of x-mV level.

- if the ramp up duration is short, the error/offset can be calibrated out with software.

*** So the trouble is the duration is too long and not feasible for possible calibration.

See attached schematic of the end-application.  

Need help to identify how to reduce the noise at input and understand why the time for output ramp up is varied.
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