# Operational amplifier - why average voltage is not stright related to pulse to duty cycle?

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.

And here is a chart

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.

• Looks (superficially) linear to me. There's an offset, but it's still linear. What output do you get with a 0ns "pulse"? – brhans Feb 24 at 13:00
• I was going to say the same as brhans, your results seem linear, thus the question doesn't really make sense. You are maybe wondering why there is an offset? – Damien Feb 24 at 13:12
• But there in fact it is about 0,37V I only see a blue curve varying between -80 uV and +800 uV. Where is your 0.37 V coming from? Your input current is 100 uA, your OTA has a transconductance (at DC) of 1 kohm (Rf) so at most you would get 100 uA * 1 k = 100 mV at the output of the opamp. What if you make Cf much smaller like 100 pF instead of 1uF? – Bimpelrekkie Feb 24 at 13:22
• Thank you for yours answer. I was not quite clear. I can not use 1uF, becasue there is only 200 pulses, and for the 1uF capacitance time delay is too long. – mil Feb 24 at 13:44
• Don't know if this accounts for your non-linearity, but OP27 has a slew rate of 2.8V/uSec. – Math Keeps Me Busy Feb 24 at 14:07