Howdy!
I'm making a detector array for a senior design project, and I'm looking for a little more advanced circuit knowledge than what I have.
We are trying to find the center of a laser beam spot for alignment purposes.
Here's the circuit we designed (one pixel shown): 
The signal from the photodiode is amplified by the op amp, then gets stuck in the capacitor to be read through the analog multiplexer by the ADC on our Arduino (hopefully no significant leakage from the switch, MUX, or otherwise). The Arduino will cycle through the "pixels" then reset the values by draining the capacitors using the MOSFET switch on the circuit.
The pulse length should be about 7 ns, so we have picked the ADA4857-2 as our op amp (high speed, less IC's). We don't care about an absolute measurement of the intensity on any one pixel, but the relative intensities are important so that we can find the center position of the beam.
My questions are:
Is this circuit sane? What is typically done in this application?
How do I calculate a value for the capacitor?
How do I minimize drift/leakage over the time between when the pulse arrives and when the arduino gets around to reading the values?
Thanks for any help you can give!