Electronic – Transimpedance Amplifier Photodiode

operational-amplifierphotodiode

I have a 660nm LED powered in a separate circuit and am trying to build a photo diode using a transimpedance amplifier but I am extremely confused. I read all the articles on this site about them.

The photodiode is most sensitive to 660nm as well.

Found here: http://advancedphotonix.com/wp-content/uploads/PDB-C142.pdf

The circuit below seems to be a common one for converting a current into a voltage. I read I can choose a large value such as 1M ohm for Rf but I am confused where the datasheet for my photodiode comes into play. Also why is there no voltage source for the op amp? Just the op amps power alone?

Light current means its most saturated current and dark is the current when there is no light correct?

Any help would be great! Photodiodes are much harder than I expected.

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Best Answer

Bottom line: what are you trying to detect - that governs how much or how little of the data sheet you need to use: -

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  • Short circuit current - what it says on the tin - how much current you could expect to see under certain lighting conditions.
  • Dark current - how much current you might expect to see in dark conditions. This will roughly double (or more) every 10degC rise in temperature and will add an offset to the output of your TIA. For instance a dark current of 10 nA at ambient might be more like 40 nA at 45degC and this through a 1Mohm resistor produces an output error of 40 mV.
  • Shunt resistance - I expect it's useful to know it is rather large in a lot of applications but some slow speed (and sensitive) applications might not be able to live with 500 Mohm.
  • Junction capacitance - books can be written about this and the interaction with noise in TIAs in higher speed applications. Basically, if you need high speed and decent sensitivity junction capacitance is usually the limiting factor.
  • Spectral range is fairly self explanatory
  • Breakdown voltage - if using the device with reverse bias (to cut-down on junction capacitance at the expense of dark current) then you need to know the limits of the device.
  • Noise equivalent power - it's basically telling you that the output noise produced when light is not hitting the device is equivalent to a certain power density.
  • Response time - it's usually all down to the junction capacitance and the load resistor so 18 pF and a 1 kohm load give a CR time of 18 ns. That is a 63% change as per the exponential curve: -

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However, what the data says is that 80% is accomplished in 50 ns and you'd expect 80% (as per the curve above) to be done within 1.5 time constants (27 ns) but, clearly there are other factors at play to make it 50 ns. Anyway it's going to help you understand how fast you can expect the output to change and is important if using it for receiving data in that sort of application.