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I want to use a photodiode in photovoltaic mode with a TIA amplifier and be able to set the gain with a feedback resistor and also minimize the dark current offset related and with certain linearity.

Abut the input light source: The input to the photodiode can be either CW or a pulsed light source with high repetition rate where I can adjust and attenuate the light intensity ect. The pulsed monochromatic light input is a pulse train with 1ns duration with 333ns pulse width.

About the photodiode: The photodiode I have is not for capturing such fast measurements. It has quite high junction capacitance between 4nF to 6nF and shunt resistance Rsh>25k.

But in my case, I don't need to capture the pulse shapes at the opamp output. I'm only interested in the average current produced by the photodiode, meaning that I will calibrate the input light average versus the opamp DC output voltage. So that I can obtain some relation between average light input power to opamp output voltage.

There is too much theory in stability and GBWP of the TIA amplifier and most of it I believe is written for fast diode applications. But I'm still unsure about how I can achieve a DC response for such input as described without causing stability issues. So basically all I want is to reduce the opamp BW, meaning that I want it to act as a filter for the input and to obtain average of the input as DC voltage. And input/output response can be very slow like few milliseconds.

I can say that I want to set the Rf as a trimpot but I think it will be roughly 50k since the average current I expect is more or less 50uA so that I can obtain 2.5V to couple it to a 3.3V ADC of a microcontroller.

As for the opamp, I want to either use this or this unity-gain stable opamp since I need to use with 5V single supply and rail to rail.

Here is the TIA with photodiode:

enter image description here

But the above requires to set the correct feedback capacitor value Cf and also requires some sort of modification for my purpose(to reduce the BW of the TIA to obtain DC average only).

Adding a cap between the non-inverting input can cause stability issues? And if I instead make Cf very large like 1uF would that cause stability as well? And if I do the averaging after the TIA output then wouldn't the opamp have to be superfast type since I have fast light input?

So the bottom line is, my question is how to estimate the Cf in my case and how to modify the opamp to obtain DC average at the output proper way? Does any of the opamps I have have the right specs for the purpose?

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2 Answers 2

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Let's say you want a millisecond time constant. RC=1ms, R=50k, so C=20nF. That's plenty for stability with just about any unity-gain stable opamp.

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  • \$\begingroup\$ By C, you mean Cf? And Cj has no effect on time constant or BW? \$\endgroup\$ Commented May 6, 2024 at 18:47
  • \$\begingroup\$ @cm64 Yes, Cf. Ci affects noise and stability. \$\endgroup\$ Commented May 6, 2024 at 20:06
  • \$\begingroup\$ Opened a new question about it here electronics.stackexchange.com/questions/714213/… \$\endgroup\$ Commented May 23, 2024 at 18:08
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But the above requires to set the correct feedback capacitor value Cf and also requires some sort of modification for my purpose(to reduce the BW of the TIA to obtain DC average only).

The compensation (feedback) capacitor Cf has to have a certain minimum value for stability reasons. You can always choose to overcompensate the TIA to get the bandwidth down to whatever you need. The following example has an overall BW = 100Hz by making Cf = 27nF. The additional inside the loop RC filter (220 Ohm + 100nF) together with the associated 5.6nF compensation capacitor is strictly not neccessary but enhances the attenuation at higher frequencies. The output filter can also be placed after the TIA or can be left out altogether.

enter image description here

As per request the above circuit split up into simple TIA + lowpass buffer stage:

enter image description here

enter image description here

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  • \$\begingroup\$ Is it possible you provide two options in different schematics? \$\endgroup\$ Commented May 7, 2024 at 12:33
  • \$\begingroup\$ @cm64 * What "options" are you referring to? Without additional in-loop filter? With passive, external filter? \$\endgroup\$ Commented May 7, 2024 at 20:53
  • \$\begingroup\$ Yes I mean please show both circuits(simplistic one and the one with in-loop filter) in different schematics. At the moment wo circuits are superimposed into one. Could you do that? \$\endgroup\$ Commented May 8, 2024 at 11:04
  • \$\begingroup\$ @cm64 * Added requested circuits. \$\endgroup\$ Commented May 11, 2024 at 4:28
  • \$\begingroup\$ Opened a new question about it here electronics.stackexchange.com/questions/714213/… \$\endgroup\$ Commented May 23, 2024 at 18:08

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