I've been messing around with photodiodes do measure relative light intensity from very low light to very bright conditions. My search for information on photodiodes and the circuits they are implemented in led me to various different configurations.
I understand that photodiodes are run in reverse bias and are treated as variable current sources. A seemingly very common measurement method is to use a transimpedance amplifier with the photodiode feeding directly into the op-amp's inverting input. Based on the Wikipedia’s article on photodiodes, it would seem that this is the kind circuit one would want to use for measuring light because it supposedly has less noise. I've tried this configuration and I will touch on that in a bit. Continuing, with the use of the transimpedance amplifier, there are at least two different ways to bias the photodiode. One of the methods involves imposing on the cathode a positive voltage relative to the inverting input with the anode feeding the inverting input. A second method is to impose on the anode a negative voltage relative to the inverting input and tying the cathode to the inverting input.
An alternative to using a transimpedance amplifier would be to follow an approach similar to that suggested in the product datasheets for the photodiodes from Thor Labs (shown below) where there is a load resistor between the cathode and anode, and then the potential across the resistor is measured. This voltage can be measured directly or indirectly.
I am generally aware of the effects of biasing, but I have not been able to see the benefits of doing so in my own prototype circuits.
When I used a transimpedance amplifier (AD8015) as a first stage and an OP196 as a second stage differential amplifier, it seemed that the photodiode had two states, on and off. At certain medium intensity light levels the circuit did pick up a bit of light, and high intensity it went to full on immediately. At really low level light conditions, it couldn't measure anything. My circuit at the time couldn't go lower than 0.5 V bias.
I had a different photodiode circuit that use a load resistor to compare against, and it detected the ramp up of light intensity sooner than the transimpedance circuit. This meant that it could see the transient response and seemingly higher resolution. The loaded circuit could also see low to medium intensity light in situations where the transimpedance circuit could not. However, once the light intensity hit a certain point, the photodiode behaved like an avalanche diode despite being a PIN diode. The transimpedance circuit had a very rapid and high amplitude response (i.e. it clipped on my 0-10 V output). I have no need to have high frequency response (<10 Hz is acceptable). The transimpedance amplifier circuit had a smaller window of detectable light intensity that was significantly smaller than the loaded circuit and had next to no discernment in light intensity.
I have since replaced my two stage transimpedance circuit with a single stage differential amplifier measuring across a loaded photodiode. What I found is that a large load resistor (1.8 kΩ) introduces a rather high noise floor. By reducing the resistor size (100 Ω) it initially reduced the measured potential across the load. Reducing the load further (50 Ω) cause the potential to increase, and then reducing even further (10 Ω) caused a massive decrease in potential. The first step to 100 Ω immediately removed the high noise floor.
So, my questions are as follows. What are the pros and cons of connecting the anode to the transimpedance amplifier versus connecting the cathode to the transimpedance amplifier? The datasheet for the AD8015 speaks a bit to this, but I believe it was truly intended for fiber optic communications. Why does the transimpedance amplifier seem to have no middle ground in terms of detecting light intensity? How does the load impact the photodiode (i.e. how does one determine optimal load, what determines a photodiodes ability to drive a load)? Can I get a better explanation as to when one would use a loaded circuit versus when one would use a transimpedance amplifier? Is there a way to modify a transimpedance amplifier circuit so that it has better discernment in light intensity?
These photos are referenced from the datasheet for the Analog Devices AD8015.
This photo is the standard circuit used in photodiode datasheets from Thor Labs. This is the circuit I'm using currently except that I have a differential amplifier across the load resistor. My load resistor is 50 Ω.
This is the link to the AD8015 datasheet. AD8015 Datasheet