Choosing a photodiode - Page 1
Forum section: electronics / beginners

author: abhaymv
author_rank: Contributor
timestamp: July 22, 2013, 05:45:32 pm
content: Hey all!So basically I need a photodiode that gives a current output for a spectrum ranging from 300nm to 1100nm. I've seen some photodiodes whose response peaks at near 900nm. Now, I need to sense the short ckt current from the photodiode and send it to matlab by some ADC. I'm guessing I'll need an instrumentation amp here?Most photodiodes only give a relative spectral response, andI want to know specifically the value of the spectral response (The exact A/W) for that wavelength.I intend to measure the current to reverse calculate the radiant flux (in watts) and use the photodiode as a sort of radiometer. So what kind of a photodiode should I look for? Will any one do? The flux level falling on the diode would be something like 100 W/m^2.

author: Odysseus
author_rank: Regular Contributor
timestamp: July 22, 2013, 09:23:45 pm
content: Quote from: abhaymv on July 22, 2013, 05:45:32 pmNope, just any normal op-amp configured as a transimpedance amplifier. Don't quote me on the correct orientation of the diode. Just try both polaritiesAlso, of you are sensing light of a wide dynamic range, consider using a diode instead of a resistor in the feedback loop. You'd be surprised how little light is detectable this way. But since this will result in a logarithmic response, it may not work easily if you want to do accurate calculations based on the output.You can also try combining photodiodes that cover different wavelengths in order to get the response you want, like a visible in combination with an infrared photodiode in parallel. Gain matching will likely be poor, but I'd consider it worth a shot.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 22, 2013, 09:31:01 pm
content: If I understand your question correctly, then there is no easy or inexpensive method for doing what you need.The problem is that not only is the photo diode sensitivity not constant over the wavelength range of interest, neither is the average light source of interest.You would need a very narrow yet tunable bandpass filter in front of the IR diode to make the math work. If you had that, you could create an accurate per-device calibration curve for the photo diode, similar to the one seen in the datasheet. Once you have the calibration data, you can sweep the filter in front of an unknown IR light source, and obtain the *combined* response curve of the light source and the detector. It is then a simple mathematical operation, using the known calibration curve for the detector, to calculate the total incident energy per surface area.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 23, 2013, 12:23:43 am
content: What absolute accuracy do you need?Is your source going to be a collimated beam or a diffuse source like a lamp? Is it spectrally coherent (like a laser), incoherent but narrowband (several nm), or broad band (100s of nm)?Generally speaking, calibrating a photodiode for absolute efficiency is quite difficult. It is rarely specified for the bare diode because it depends on a number of other factors. The bias voltage and the incidence angle of the light have a noticeable effect. Also, in real applications you usually have some sort of filters or lenses in front of the diode that affects the system quantum efficiency, so absolute calibration usually only makes sense for a system.