ADA4528-2ACPZ exposed pad

Hi, I am using ADA4528-2ACPZ in a design where The exposed pad has been connected to ground. Data sheet suggest connecting the pad to V- or to leave it opened. The circuit is simply used as a buffer (non inverting gain 1). However the input voltage is attenuated at its output to +/- 40mV peak. In put is +1V peak sinewave with frequencies swept from 1 to 1000Hz.

I have got the ground connecting vias disconnected (by pcb manufacturer using back-drilling technique used on high speed pcb designs). The results is the same. The power rails are +/- 2.5V.

Could the cip be damaged?

With thanks and regards, Vic