I've recently built some fiber-optic bend sensors and I want to read the values I get from them into a computer via an Arduino. I'm measuring the light with this photodiode from Industrial Fiber Optics. Currently, I'm giving the LED on the other end as well as the photodiode 2.2V. My question has to do with the fact that the voltage fluctuations as measured by a multi-meter on the photodiode are linear, but rather small as the fiber is deformed, even quite radically. With the fiber straight, depending on the fiber (its hard to score them identically), the voltage hovers around 1.92V, for instance, and with bending it will it will go up to, say, 1.93-1.94V. I'm not worried about getting the voltages identical as I can scale in software.
What I am worried about is losing resolution when doing A/D with the Arduino. If my voltage fluctuations are on the order of 10mV, won't the Arduino's 10-bit A/D quantize the hell out of it, even if I step the voltage up to 5V with a voltage divider? What I'm looking for is an analog scaler. How can I stretch out that range between 1.92 and 1.94 to cover the full range, from 0V to 5V so that I can take advantage of the full range of the Arduino A/D?
I feel like this has got to be a common operation in electronics, but I've never studied it formally, so a lot of things are lost on me.
(You may be thinking, as davr was, "why are you using fiber optics for bend sensing? Why would you expect a voltage change when the fiber is bent?" The trick is to remove the cladding on one side of the fiber optic cable. This lets light spill out. When the cable is bent away from the scoring, even more light is let out of the cable, causing a voltage drop in the receiver, and vice versa.)
Best Answer
So if I understand correctly, you want to be able to "read" a 10 mV variation on top of a 1.9V signal?
If that is the case then I would suggest two separate stages. The first will be a photodiode amplifier (page 9 is the most standard of circuits). This will help to get the current from your photodiode translated into voltage.
The second stage will be an instrumentatation amplifier, such as the INA family from Texas Instruments (the best but also can be expensive). This will help to remove your "common mode" signal, which in this case is the 1.9 V. You can also add gain in to the instrumentation amp or alternately add a simple op amp in a non-inverting configuration at the end to help gain your signal up to the necessary 5 V.
I'm not saying it'll be perfect, but I think that's a good start.
As a final note, I like David's idea above about the clamps, even though those can cause some measurement errors at the A/D converter. What is more important though is if you can swing it, try a better op amp than the 741. Those are common but the specs are terrible. The 3 or 4 mV of offset voltage at the input terminals could really mess up a small signal like you're trying to measure.
~Chris Gammell