VCA821: the Bottom of my VCA821 output waveform has distortion

My test of the board shows that the waveform would not have distortioin if it output within about 70mVpp

but it would have distortion if it output too high

I have asked someone, be told that my input have offset，but the output shows that my waveform have no offset ,just  the buttom of the waveform is cut

Thanks a lot