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I previously asked about a light detection circuit and was set along the path of using a photo diode.

I have a simple circuit created, now I need to create a null or offset so I can gain the PD at 0V DC dark and 10V DC light. My photo diode outputs about 200 mV under my dark condition and approx 300 mV under my light condition (100 mV delta).

Can I do anything to improve the circuit using the current components or do I need a different Op Amp?

Circuit

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I do not recommend using the PD in a photovoltaic mode if you want the microsecond response times implied by your previous question. Constant negative bias is preferred because it allows you to avoid the slow response caused by the PD capacitance.

The 200mV is because the relatively large bias current (hundreds of nA probably) of the op-amp is flowing through the PD. It will change a lot with temperature. A lower bias current op-amp would reduce the voltage, but it will still be very slow.

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There are plenty of photodiode circuits to see if you googled but I have to admit, the one you have used is not one of them. To get speed and gain try this circuit and be prepared to modify the value of the feedback resistor (R\$_F\$) to suit the optical device you use: -

enter image description here

The LT1880 data sheet has a couple of design ideas for using photodiodes that are possibly useful. Should you decide to use a pulsing light source it should be OK up to 10kHz.

The output voltage will rise in the presence of higher incident light and, I believe, choosing the right op-amp will allow you to avoid using a negative voltage rail. Look for rail-to-rail devices.

Regarding your previous question, I made some pointers about controlling the light source amplitude and using a pulsing light source. I don't know if you have thought about that much?

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  • \$\begingroup\$ I will need to order some components this week. I have thought about the light source variable and on the bench I see a response difference from fluorescent and incandescent light. My end use application also has adjustable pulse strobe lights, so I have lots of options to experiment with light. \$\endgroup\$
    – Tinkerer
    Mar 16, 2014 at 19:10

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