Optical displacement sensor noise and current to voltage conversion
The electrical interference from all sources ( line E field, stimulator, SMPS noise, AM RF signals ) is expected. So use twisted pairs for everything.

A small RF cap used to get rid of AM RF is optional as Photo Diodes (PD) already have this capacitance.

The Load R determines signal gain and rise time could be 10k to 1M depending on stray light blocking ( optical apertures), proximity and reflection coefficient.  

I estimated 2% reflection in my design which is unknown without all physical experiment assumptions given in question.

This design differentiates the signal into a comparator by taking the difference of an integrating RC filter.  So the actual DC voltage does not matter (0~3V) as long as there is the reflection pulse and not crosstalk from the stimulus pulse.  OK?

I do not recommend this sensor and instead use the Vishay 5mm smallest angle emitter (&lt;10') and Photodiode with black filter.

OK   any questions? Here is the sim.

I simulated steady DC LED light with R1 to indicate reflected signal only !!  Reduce R2 if voltage >3V. It can sense near 0V down to 1mVpp ok with care on noise. 



Use heatshrink tubing over 5mm LEDs to block all stray light and focus only on target then bundle together Emitter/Detector as a pair and separate,shield RX wires from noise.

====>   &lt;==== small aperture &lt;3mm with heat shrink tubing on 5mm LED , PD.

EMitters   OP297A or equiv
PIN PD = SFH213FA  {Mouser}  or equiv