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:


simulate this circuit – Schematic created using CircuitLab

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.

  • \$\begingroup\$ is the opamp oscillating? is your scope capable to see such? \$\endgroup\$ – analogsystemsrf Nov 14 '19 at 2:26
  • \$\begingroup\$ @analogsystemsrf The op-amp is not oscillating. All the scope sees is a slight DC voltage drop when the power supply is turned on. I'm beginning to suspect that it's an issue with the SiPMs because when I took off the op-amp, the results are still the same. No signal is shown even with the op-amp off of the board. \$\endgroup\$ – BestQualityVacuum Nov 18 '19 at 14:53
  • \$\begingroup\$ @analogsystemsrf Without the op-amp, I conducted a slightly crude test, shining a penlight on my SiPM array (biased at -30 V). When no light is shown, it's at -5V, but when light is on the array, my scope shows the output voltage jump to -30V, and my ammeter shows an increase in the current draw, from 5 microamps to 30 microamps. However, I'm not getting a peak that's characteristic of an SiPM, though that may be attributed to my testing procedure. It's not oscillating, but I do see a DC voltage, and it does change when light is on the array. \$\endgroup\$ – BestQualityVacuum Nov 19 '19 at 17:09

There are several issues here.

First, that amplifier circuit will not work. You have 16 diode arrays in parallel, each with 4nF of input capacitance. That is an enormous input capacitance, but you have almost no feedback capacitance. You should be closer to 310pF, not 3pF. See: http://www.ti.com/lit/an/sboa055a/sboa055a.pdf

Second, you have have +/-5 volts powering your opamp. If you are getting -30V from a -5v rail, something is very wrong. Have you burned out your opamp and/or SIPM? Or shorted something?

Third, the datasheet you linked gives best practices for building transimpedance amplifiers using that op amp, including things like decoupling capacitors. You aren't following them.

Forth, the SIPM vendor has recommended circuits for biasing and decoupling the SIPM. You aren't following those either: https://www.onsemi.com/pub/Collateral/AND9782-D.PDF

I would start by verifying that the SIPM itself is still working by testing it with a scope and no amplifier. I would then fix the bias circuit. Finally, I would fix the amplifier circuit by adding the correct feedback capacitance and decoupling capacitors.

I wonder though if a transimpedance amplifier is the right thing to use here? You are only getting 470 ohms transimpedance gain but using a detector with a gain of over 6 million. What are you trying to do?

  • \$\begingroup\$ Thanks for your reply. I apologize for not being more thorough, but to be more clear, I'm getting -30V on the circuit on the left, without the op-amp, when the SiPMs are active. I have decoupling caps for my op-amp circuit on the right, and the SiPMs are safely biased with a filter. I just didn't show it in my initial circuit, and I'm just trying to reduce clutter. With regards to the input capacitance, would that burn out the op-amp by any chance? With regards to my goal, I'm trying right now to just convert the array signal into a voltage signal via the op-amp. \$\endgroup\$ – BestQualityVacuum Nov 21 '19 at 15:39
  • \$\begingroup\$ Good, with the SIPM filter installed it will be almost impossible to break the SIPM. You're showing 6.8uF. Those are in addition to the recommended 0.1nF caps right? Without the larger feedback capacitor the circuit should behave as a few hundred MHz oscillator, but that won't break the amp. The usual way these amplifiers are broken is that the input voltage or power supply voltage exceeds +/- 5v. In this case, since you have a 30v bias voltage, it was probably connected to the amp accidentally. If you just want to create a voltage signal, why not the resistor circuit? \$\endgroup\$ – user1850479 Nov 21 '19 at 16:33
  • \$\begingroup\$ The -30V bias and +/-5 volt pins were isolated and not shorted together, but I wouldn't rule out the possibility of them somehow accidentally shorted together. I think that perhaps, some of the SiPMs were not oriented correctly where their 'fast out' pin was biased instead of the anode. Faulty orientation and whatnot, but that doesn't explain why I'm not getting any signal at all, no oscillations or anything. Regarding the use of a resistor circuit, we want a positive signal, so the TIA helps convert our negative current signal into a positive voltage. \$\endgroup\$ – BestQualityVacuum Nov 21 '19 at 17:49
  • \$\begingroup\$ Putting the sipm in the wrong way will expose the amp to the bias voltage, destroying it and possibly the sipm (although the bias filter will hopefully save you here). If you just want an inverting amplifier, you can buy a ready made evaluation board from digikey for less than it's going to cost you to make this work. \$\endgroup\$ – user1850479 Nov 21 '19 at 18:09
  • \$\begingroup\$ If you don't mind me asking, what's the best way to visualize the effects of changing the feedback capacitor in the TIA with MATLAB? I was directed to this link from a previous post, and I used it to visualize the gain/freq. plot to see how the cap affects the gain. It looks like it just makes the gain at the phase shift smaller, but what other factors of a TIA should I look at if I were to adjust the feedback capacitor? \$\endgroup\$ – BestQualityVacuum Nov 25 '19 at 16:48

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