HELP: an active high pass filter

[sxb416]

HELP:high pass filter

I need a active high pass filter.
It is fully differential mode filter and first order.
when I simulate the opposite phase amp,the amplitude is -r1/r2.
but if I add the cap before the resistor,it's amplitude is -40db，
why??

 

[JoannesPaulus]

HELP:high pass filter

maybe you should tell us what topology you are using, what frequency you are considering, what size capacitor and so on...