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This explanation assumes a basic understanding of operational amplifiers. If you are unfamiliar with these, here are some good resources: this is a pretty decent overview

Circuit schematic, descriptions below explore each box in order

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1. SiPM - Silicon Photomultiplier, essentially a bunch of tiny photo diodes (reverse of an LED, generates current proportional to light) in a big array.

   1. Introduction to SiPM from ONSemi - really good resource that explains operation of these sensors

      1. https://www.onsemi.com/pub/collateral/and9770-d.pdf

2. Transimpedance Amplifier - Converts current from the SiPM to a voltage

   1. Ideally Vout = R1*Iin, where Iin is the current from the SiPM, and R1 is the resistor in the feedback path of the op-amp.

      1. This is just Ohm’s law, you can theoretically get the same behaviour by putting a resistor in series with the SiPM. However, the op amp isolates the resistor from parasitic - capacitance from the SiPM, input resistance into the next component in the circuit, etc. Using a transimpedance amplifier is a much more dependable and consistent way of getting a voltage from a current source.

   2. The capacitor C1 in the feedback path limits the bandwidth of the op amp to act as a filter

   3. Resource about capacitors in transimpednace amplifiers:

   Resource about op-amp bandwidth

1. Fantastic resource that explains transimpedance amplifiers and some of the issues with them

   1. Transimpedance amplifiers for SiPMs (starts pg. 7)
      

2. Bandpass filter - very simple passive filter to reduce noise

   1. resource explaining bandpass filters

3. Amplifier - second stage amplification of the signal. You cannot amplify all the way in one go due to bandwidth limitations. This also allows the signal to be filtered before full amplification, which is better for signal/noise ration.

   1. Explanation of simple non-inverting op-amp

   2. technically a better way of doing this would be to use an inverting op amp with the filter built in, which gives a much sharper cut-off of the filter. However, that would require using split supplies, because it would invert the signal. This would have significantly increased the complexity of the power supplies. Inverting op amps generally have better characteristics than non-inverting ones.
      It likely would have been better to add another capacitor in the feedback path of this op-amp as well, I am not sure why that wasn’t done.

4. Peak detector - this elongates the signals so the ADC has more time to complete it’s conversion.

   1. This likely would have been better accomplished using a sample and hold circuit - there might actually be one built into the PIC18F, but that option was unknown when this was designed

   2. resource explaining peak detectors

5. Buffer - to avoid unwanted parasitics from the unknown impedance of the MCU pins and potential interference with the ADC conversion, the buffer isolates the circuit from the MCU

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