AD835 Output is non-linear


I am using AD835 as a mixer for mixing two sinusoidal signals. One of the signals is 800mV and the other is 1V. The 800mV signal is coming from the output of an Op-amp. When I change the value of this voltage (for my application) the multiplication does not follow a linear trend. Can somebody please help as to why I am encountering this non-linearity? Thanks.