I'm working on a transimpedance amplifier circuit, but it appears that there are issues reading out a signal. The source is comprised of sixteen individual SiPMs in parallel with each other, specifically the MicroFJ-60035. For this array, I'm running two variants of the circuit: one array without an op-amp, and one with an op-amp. Below is a schematic showcasing both types:
Apologizes for the formatting, but I thought that it would be easier to showcase the two types of circuit in one schematic rather than have two large schematics take up the page. While I only show one SiPM in the schematic, you can think of it as sixteen SiPMs all connected in parallel, with the anodes and cathodes of all sixteen parts connected with the others. Please note that the two circuits shown are independent of each other, being on separate boards and all.
Now, the first schematic, shown on the left without an op-amp, has no issues with it. Rather, it functions as expected given the array configuration and the resistor, showcasing a negative signal at the oscilloscope. However, there are issues with the transimpedance circuit on the right. The op-amp I'm using for the circuit is the TI OPA-656, and the output of the circuit is not showing anything even when light is shining on it.
At first, I thought that the issue was just bad orientation of the SiPMs. So, I've unsoldered the SiPMs and made sure that they were in the correct orientation. Doing so did not solve the issue. I'm beginning to wonder if it might not be an issue with the layout, but an issue with the op-amp used in the circuit. Having sixteen SiPMs in parallel increases the capacitive load at the inverting input, but would that alone prevent any output from showing at all? I would have expect some response, but to have no response is strange.