We figure out the offset issue of the chip AD 734. We measured the noise of chip AD 734 with 50 ohm input terminated.
The noise is shown below for two chips we are using and noise from spectrum analyzer is attached for reference.
The noise of AD 734 is about 1 uV/root(Hz) at around 50 Hz. The sensitivity of of the chip should be:
dV*dV/10 = 1 uV/root(Hz) => dV ~ 3 mV/root(Hz)
We are not sure about that we understand the noise propagation through the chip correctly.