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OPA355 Circuit - Impact of resistor

Frank QT
Intellectual 350 points


I have the following circuit

It is essentially a AC amplifier. I have a signal that is 500KHz and I am able to amplify even a few nA signals with good accuracy. The problem is the sun. When device is  working outdoors, the photodiode (shown as a current source) becomes forward biased. The current can reach to a few mAs under adverse conditions and at that moment I loose the signal.

I like to increase this circuit without impacting its gain and noise performance. I have considered the following:

- Reduce the (R2) 20K resistor to 2K, this would reduce both shot noise and voltage drop across resistor and make it more noise tolerant. When I try this at the bench, I run into issues and I cannot understand why? Appreciate help from the masters 

Thx

  • 0
    TI__Mastermind 22825 points

    Hello Frank,

    Is there any kind of optical shielding you can do to the photo-diode so that the sun's light power won't overwhelm your circuit?

    Your post mentions modifying the circuit to reduce shot noise. However if your issue is circuit overload when placed under the sun, I doubt if you need to modify the circuit for "random noise" phenomenon! Shielding, reducing the I-V gain, and adding an over-voltage clamp are the only things that may have a chance, in my in-experienced view.

    Regards,

    Hooman