AD8601 unity gain buffer

I have applied a signal to the input of the AD8601 which is an exponential function(like discharging and charging waveforms of an RC filter). The signal oscillates between 1.5V and 3.5V @ 800kHz. The AD8601 is in differential configuration and the resistors in the configuration is 100K all. The output is single ended and fed into a comparator. I want output signal to be exactly the same as the input(like a unity gain buffer) but what i get is a triangular signal rather than exponential which has a slightly less frequency e.g.780kHz and slightly have larger peak to peak value and probably has a phase difference(I did not measure the phase difference). I've used plain differential configuration with 4 resistors. AD8601 is fed from 5V supply. What am I doing wrong? Is this part suitable for my purpose? Thanks.