I have a system which demands optical data transmission since rotating parts are present. A mixed mechanical-electrical engineering project has led us to use photodiodes to receive data from one rotating end to another. The communication system is through a CAN bus. The transmitter and receiver diodes are:
In a nutshell: most applications I have thought of and tried don't seem to comply with the speed I'm looking for, which is at least 300 kHz.
The circuits I have tried are the following
Circuit 1 - A simple "RC" (the C stems from the photodiode junction capacitance) circuit as a means to take the output of a square wave. The problem arose when I looked at the scope display (image right below the schematic) and the turn-on and off time were way too high for my implementation as can be seen below.
simulate this circuit – Schematic created using CircuitLab
The scope associated to both input (yellow square-ish wave) and output (green RC-like response) is shown below:
The operating frequency is 206 kHz, and the response barely reaches the 12 and 0 V level. The lower boundary is at 4.8 V and such delayed reponse might disturb the communication system. If R is increased the time delay is of course even greater, however, by reducing it there's an obvious loss of voltage swing for a comparator (e.g. LM311N) to sense.
Other circuits I have tried are the following:
Circuit 2 - Q1 and R1 sources current towards the photodiode in the range of μA, which is pretty much the order of magnitude from the datasheet, so that when the photodiode is subject to the specific light source, the current amount is increased and voltage could be sensed better. Despite the attempt, no clear result came out and the BW was limited to 10 kHz without complete signal distortion.
Circuit 3- The idea has been to push current towards the transistor base and then switch the output so as to find a better result in the end, which nonetheless didn't work quite well, and the bandwidth was again reduced.
All circuits share the same input method.
I was thinking of the basic transimpedance amplifier method, however, I wouldn't expect an op-amp to be faster than a single transistor network, not to mention the additional PCB space it would take compared to a transistor.
What could be done to improve the system and meet the 300 kHz requirement? Any ideas would be really appreciated.