My formal education was in mechanical engineering and I’m well trained in some CS as well. Yet here I am on an electrical project (that will eventually turn CS). I have been lost and confused for many months now and could really use some specific help.
Project scope/goals: Measurements from the sample to photodiode should be in the 0.3 to 3 picoAmp range. An array of photodiodes will detect this small amount of light. Right now I’m just trying to get a circuit working for a single photodiode at that extremely low light range. Time response can be long or short because we can hit the sample with the laser for a long time without any problem. The laser rep rate is ~76MHz versus the approx. detector bandwidth ~23KHz so there shouldn’t be a problem with signal decay.
- S10355 photodiode (we are looking to replace, suggestions would be awesome)
- Powered breadboard with +5V, +15V, and -15V supplies
- NI USB-6211 for measurements
- Op Amps we have around(happy to buy more as needed)
- TLC271 (0 to +15)
- TL031 (-15 to 15)
- MCP603 (0 to +5)
- Large resistors: 10Meg, 22Meg
- Resistor kit (100 ohm to 900k ohm, lots of values)
‘Ideal’ Circuit simulation (that doesn’t really work but is good for visualization):
Adapted from book:
I chose my resistance total based on amplifying 0.3picoAmps to 1 Volt. The book also describes a way to do this with a single op amp but I haven’t gotten that one to work even a wee bit. The only other way seems to be a tee-network but this causes proportional noise gain as well which wouldn’t be appropriate for my application from what I can tell.
Major problem: There appears to be so much noise even at ~no light~ conditions (there’s never no light but I can get close), that the voltage readings are maxed. Since we are looking at subtle changes this makes it pretty unclear. I read about making a pseudo-ground for the cathode of the photodiode and set one up at 1V from the +5V of the breadboard. This doesn’t seem to do as the forum post indicated or maybe my gain is so high I need a mV or pV pseudo-ground? Not sure. I have also read that choice of Op Amp is critical, and I’m not sure I have the right ones for the job right now, could this be hindering linear response and ground levels?
Power turned off/no light Vout:
Power turned on/no light Vout:
Future problems: I know to eventually implement any of this on a PCB board (for the full array of photodiodes) I will have to be exceptionally wary of noise for detecting such low current levels. Do you have some tips for this? There’s a nice noise analysis in the book but I don’t fully understand the ee concepts I am working around to be honest.