The PSoC4 OpAmps have a offset voltage of 1mV. So I guess you need an external OpAmp with much better specifications, since otherwise the offset will make your measurements completely unusable.
Consider adpating CDS to your problem -
http://www.cypress.com/?rID=49159 AN66444 - PSoC® 3 and PSoC 5LP Correlated Double Sampling to Reduce Offset, Drift, and Low Frequency Noise
http://www.cypress.com/?rID=2894 AN2226 - PSoC® 1 - Using Correlated Double Sampling to Reduce Offset, Drift, and Low Frequency Noise
http://www.cypress.com/?rID=2813 AN2099 - PSoC® 1, PSoC 3, PSoC 4, and PSoC 5LP - Single-Pole Infinite Impulse Response (IIR) Filters
Here is a solution using an OpAmp with much more offset, which may be irrelative
to the design depending on how you are using PIR -
Good Day to you!
Thnak you so much! The links are really useful, thanks for your support!
Glad to have been of assistance.