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I'm currently working on a project that involves converting very low photo diode current pulses, typically around 100nA, into digital pulses. The pulse-repetition frequency (PRF) is about 10 kHz, with a pulse width of approximately 20 μs.

Considering the setup, I'm contemplating using an immediate Transimpedance amplifier followed by a limiting amplifier and Comparator. However, I'm facing some uncertainties regarding the design approach i am taking.

Given the potential noise issues, primarily stemming from the use of a DCDC that might introduce unwanted noise, I am planning to set the threshold at 500mV. This threshold will serve as a reference point for the system to effectively manage any potential noise during the amplification and conversion process.

For the limiting amplifier to achieve an output of 500mV with a gain of 10V/V, the transimpedance amplifier (TIA) output should be 50mV, implying the need for a minimum gain of 500kΩ. However, I'm unsure about the viability of such a high gain, as it seems to lead to a very low feedback cap.

In light of this, I'm considering whether using an instrumentation amplifier instead of a transimpedance amplifier would be a better approach for this specific setup. I'd appreciate any insights or advice from the community on the most suitable way to proceed with this design.

More Details: Light spectrum is 1064nm and Responsivity of photo diode is 30-50 A/W(depends on temperature), its is an APD, the incoming light is as low as 1mW/m2

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  • \$\begingroup\$ If the purpose of device is to convert light impulses to digital (logical) impulses, then role of ADC is unclear. Could you specify? Do you have at least draft of block diagram? It is possible that backlight will interfere with signal and circuit will need a bandpass filter or other technique to avoid it. \$\endgroup\$
    – Vladimir
    Commented Oct 14, 2023 at 20:06
  • \$\begingroup\$ Have you picked out the diode? Usually active area, wavelength drive selection of a diode, then diode capacitance and dark noise drive the electrical design. \$\endgroup\$ Commented Oct 14, 2023 at 20:39
  • \$\begingroup\$ @Vladimir corrected ADC to Comparator, i mean a sigle bit ADC here, anyway corrected it \$\endgroup\$
    – kakeh
    Commented Oct 15, 2023 at 6:01
  • \$\begingroup\$ @user1850479 diode is C30955EH \$\endgroup\$
    – kakeh
    Commented Oct 15, 2023 at 6:01
  • \$\begingroup\$ That's an avalanche diode rather than a normal photodiode and it has 100nA of typical dark current, which will make detecting 100 nA, 20% duty cycle pulses difficult. I'd probably choose a different detector, or else look into optimizing the dark current as much as possible. \$\endgroup\$ Commented Oct 15, 2023 at 13:49

2 Answers 2

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Your question is a bit unspecific in some point, so is my answer (E.g. what is the light source spectrum and photodiode sensitivity, etc.)

Don't care too much about the DCDC converter. This noise source can be handled well with proper layout and decoupling techniques.

The question is the noise picked up by the photo diode from ambient light. 100nA is a very faint signal.

A transimpedance amplifier is the right thing to use here, because it converts current to voltage, which you are going to process with your ADC. An instrumentation amplifier converts differential voltage to voltage, which is not what you need in your case.

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  • \$\begingroup\$ corrected Question with adding more details in bottom \$\endgroup\$
    – kakeh
    Commented Oct 15, 2023 at 6:02
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Light to frequency: Take a CMOS 555 and replace Ra with the photo-diode.
The time the output is high is set by (Ra+Rb) & C. The low time is set by Rb & C. With Ra being a current from the diode the high time is dependent on light. There are different types of CMOS 555 but you want one that has very little input current.

With no light the leakage current is very little, and the charge up time will be very long. (diode pints up) enter image description here

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