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Several questions about precision amps for photodiodes

thomas.missbach
Prodigy 95 points
Other Parts Discussed in Thread: LMP7721, OPA381, OPA350, OPA2277, OPA335

Hello to all,

I have several photodiode-specific questions. My application is a position detector based on a lateral effect diode. For me it is very important to have nearly zero offset und strong liniarity over wide temperature ranges used in a low frequency range < 10 kHz. The measured current is between 100 nA and 500 µA.

At the moment I use auto zero / zero drift operational amplifiers and the lateral effect diode is reverse biased to improve linearity. But there are some additional noise in the signal. I guess it comes from the auto-zero-chopper (typ. 1.2µVpp). Is there any other applicable solution having nearly zero offset (ideally <5 µV and zero drift) without chopping and dual supply operation (ideally SOT-23)? Which OPV could you recommend for the use as a outputbuffer in combination with secound order butterworth to eliminate unwanted noise >10 kHz and to drive the cable capacitance of a 25 m shilded signal cable with a 10 mA load?

How should I handle the tradeoff between a low input bias current at one and the offset voltage at the other side in general? Is it in the case of a in photovoltaic-mode connected photodiode better to use amps with extreme low input bias current or with extreme low offset voltage if a zero output-offset is important? Because Eout=(-Isignal-Ibias)*R+Vos… Is this right?

Regards, Thomas

  • 0
    Genius 5430 points

    Thomas;

    Which op amps have you tried? Also what feedback resistor value are you using?

  • 0 in reply to Neil Albaugh
    Prodigy 95 points

    Hello Neil,

    at the moment I use one of your competitor Analog Devices called AD8638 in other projects we will use the LMP7721. The feedback resistor value is 10k.

    Thomas

  • 0 in reply to thomas.missbach
    Genius 5430 points

    Thomas;

    Try an OPA381-- it is an integrator-stabilized design that has essentially zero drift. Its data sheet has lots of information about photodiode circuits.

  • 0 in reply to Neil Albaugh
    Prodigy 95 points

    Okay thank you! I've already found and ordered this opa to do some lab-tests with different noise gain filter combinations. You're right, the datasheet is very extensive.

    Do you have some recommendations for a low offset output buffer?

    Thanks, Thomas

  • 0 in reply to thomas.missbach
    Genius 5430 points

    Thomas;

    I'm assuming that it is desireable to use an op amp capable of operating on the same supply voltages as the OPA381 so with that in mind and the fact that you will have the capacitance of 25 meters of cable on its output, you might look at an OPA350; it has the ability to drive high capacitance loads but its offset and drift are good but not as low as other choices.

    An OPA2277 is a very low offset & drift bipolar op amp that can operate on 5V supplies. It cannot swing all the way to the rail but a 10mA load should be no problem.  It would be better to operate it on higher voltage bipolar supplies, though. To isolate your approximately 2nF load capacitance (a guess based on coax) you should add a small resistor in series with the amplifier to isolate the capacitance. Try something like 50 to 100 ohms.

    A CMOS choice might be an OPA335; it has extremely low offset and drift. It can drive 10mA but you should probably add the isolation resistor to its output. It can operate on 5V supplies.

  • 0 in reply to Neil Albaugh
    Prodigy 95 points

    Dear Neil, thank you very much for your helpful answers. I guess OPA2277 fits in the best way becouse its allows me to use it with higher supply voltages. So I can support output ranges with +/- 10V.

    Merry christmas!

    Regards, Thomas