The electronics inside the PSOC can only work between the Vdd and Vss values. Typically Vss is 0V.
Therefore you are seeing what I would expect: when the input voltage goes below Vss then the circuit simply cannot follow it.
If you want to follow the entire waveform and it is 1Vp-p then you will need to bias the waveform so that it varies between 0V and 1V. You might try using a capacitor in series and then a couple of resistors of the same value between Vdd and the input pin, and the input pin and Vss which will pu tthe bias point at Vdd/2.
(By the way, I have little expereince with the PSOC devices and there may be a better way to do the biasing).
Thank you susan,
It's a good idea to change the bias. I've found that when I change the offset to 1 v, the output will be the same as input.
Thank you, Dana:)