I have a strange problem, that I cant find an answer. I have slow photodiode (about 2MHz) and fast, pulsed laser light source (pulses 10 to 500ns). Im trying to measure optical output power. To do it Im using operational amplifier and low pass filter. Pulses parameters: frequency 10kHz, pulse time 500ns. Here is my LT spice circuit.
I thought that after low pass filter should be directly linear to duty cycle. So for 10kHz, and 500ns pulse duration I have amplitude for ex 0,6V, so for the same frequency but pulse duration 250ns (half of previous duration), amplitude will be 0,3V. But there in fact it is about 0,37V. Here is example table for 100-700ns pulses, and amplitude.
Im really confused what is going on.
EDIT: I was not clear. Simulation works great, and as brhans said there is an offset. But im really confused about real circuit, that Ihave made. And there is some non linearity.