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Photodiode - op-amp questions

Michael Churchill
Prodigy 10 points
Other Parts Discussed in Thread: LMP7721, INA330, ADS1282

I have some questions about selection of a TI op-amp for a photodiode system.

Sytem details:
Goal is to measure absolute light levels (will calibrate photodiode system with known radiance standard)
Hamamatsu S1336 photodiode (0.33 A/W, Id=50pA)
Photovoltaic mode (0V bias)
Necessary bandwidth: 2 kHz
Input light levels: 1e-2 to 1e4 nW (I don't need a single photodiode to have this dynamic range, I will have several with varying feedback resistor values).

Questions:
1. Should I go for a low bias current (OPA129) or a low offset voltage and drift (OPA124)? Initially I would go with low drift, since I don't want to continually have to calibrate the system with the known radiance standard. But I'm not sure if the low bias current would be more important for the low end light input (1e-2 nW) for reducing noise.
2. Are there any other drifts I should be specifically worried about, beyond those caused by temperature variation? Do these op-amps have an offset which stays fixed if (in an ideal world) kept at a constant temperature?
3. I will have 14 different photodiodes/op-amp circuits (14 different light inputs to be imaged simultaneously). I'm thinking of putting them all on the same board, is there any benefit to having separate boards and enclosures (powered with the same power supply)?
4. I've seen the design helps in the datasheets and a couple good Burr-Brown white papers. Any other advice/tips for designing such a system?

  • 0
    Prodigy 20 points

    Waht kind of accuracy you need in low level lights measurement range?

    If you want to measure with 5% accuracy 0.01 nW you should measure with 5% accuracy photodiodes current:

    0,33 A/W * 0,01*1e-9 W =  3,3 pA

    5% from 3,3 pA = 165 fA

    Additionally, resolution is 1/10 from accuracy and equals 

    Res = 165 fA/10 = 16,5 fA

    For your purpose more effective LMP7721.

    Your photodiode system is very complex. You should use high Ohm resistors in feedback, find necessary opamps, calculate noise, using thermostability, clean FR-4 PCB and not use solder paste, which not need cleaning, and so on.

    If you really need to measure low level current, i recommend you to use S8745-01 photodiode, ADS1282 (x4 for noise reduce and sampling rate increasing), INA330 with 10 kOhm thermistor for temperature measurement and Peltier element fot photodiode temperature stabilization and several TI opamps for reference voltage filtering and TO LDO with high PSRR for photodiodes amplifier power supply filtering.

     

     

  • 0
    Genius 5430 points

    Michael;

    Which Hamamatsu S1336 photodiode are you using? There are a number of different versions of the model photodiode, varying in active area from 1.2 sq mm to 33 sq mm. These are fairly large photodiodes, consequently they have from 20 to 380pF zero bias capacitance-- a critical limitation on sensitivity-bandwidth of a transimpedance amplifier. Your SNR with an input signal of 10pW and a 2kHz bandwidth is going to be poor.

    Is it possible to use a very much smaller detector such as a  Vishay Telefunken BPW43 (1.5pF@-10V & shunt resistance of about 10G) together with optical gain from a lens or mirror?

    Otherwise, I think that you will be forced to fall back on a detector with internal gain, such as an avalanche photodiode (APD) or a photomultiplier tube (PMT).

    The suggestion of cooling the op amp with a small Peltier cooler will reduce *** input bias current  but the SNR limitation will still be the photodiode capacitance.