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Consequences of SiPMs in parallel

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LoveLaika:
I've been working on this for a little bit now, but I wanted to ask here to get some perspective. I'm working with silicon photomultiplier, and some circuits I've built work fairly well. My circuits consist of an SiPM (in this case, the MicroFJ-60035) with an op-amp in a transimpedance amplifier configuration (TI's OPA656). For reference, I've linked to two PDFs below: one showing common readout circuits and one of the datasheet of the SiPM. The TIA circuit I'm using is in the readout PDF, figure 13, left side.

Now, it works fairly well with a single SiPM, so I tried to put more SiPMs in parallel to construct arrays. However, when you put SiPMs in parallel, you increase the input capacitance and you increase the noise from each one when you connect them all to one TIA, and one output. I got some results with 4 SiPMs in parallel, but putting 16 SiPMs causes some issues. I'm not getting any output under the same test conditions. I was thinking that this may be due to the increased capacitance, but I thought the OPA656 would be able to handle that. For reference, the SiPM's datasheet states that the anode has a capacitance of 4.14 nF, so with 16 SiPMs, that's 66.24 nF. That seems quite big. Given what I'm trying to accomplish, is it better to find a different way to read out the SiPMs, or can I just simply change the op-amp to something more suitable given the large capacitance? If so, what op-amps would you recommend?


Readout: https://www.onsemi.com/pub/Collateral/AND9782-D.PDF

Datasheet: https://www.onsemi.com/pub/Collateral/MICROJ-SERIES-D.PDF

duak:
I have not used SiPMs.  I see the ap note refers to another for paralleled SiPMs:  https://www.onsemi.com/pub/Collateral/AND9778-D.PDF  Page 10 shows using Schottky diodes to isolate the SiPMs from each other but OR their current pulses together.

Without the OR diodes, when I look at how they work I'm thinking that unless the opamp can keep its summing node at zero V during a pulse, the voltage developed will allow some of the charge to get lost in the paralled SiPMs' R and C. Do you have a capacitor across the feedback resistor with a value proprtional to the number of SiPMs?  It would seem that any opamp would have to be both fast and able supply current to the summing node and if the feedback capacitance is quite large, it will need more current.  Maybe you'll need a current buffer to augment the opamp if it cannot supply the current needed.

Were you using the SiPMs in normal or fast mode? Intuitively, I don't think that opamp is fast enough for fast mode.  I'm not familiar enough with currently available parts to suggest another.

LoveLaika:
Thanks for replying. I see what you're talking about, though I wasn't using the Schottky diodes. I was just paralleling SiPMs together with the TIA circuit. I was using it with the normal signals, not the fast mode signals. Regarding what you said about the feedback capacitor, I was using the standard recommended value in the circuit, 3 pF. When you're talking about a current-buffer, do you mean a unity-gain buffer with a second op-amp? I think I can see how that may be useful in place of Schottky diodes, but it would have to be very fast. I know there are some specific op-amps developed for unity-gain applications, so I'm curious how throwing one in the mix will help. I'm not really familiar with op-amp characteristics myself. I tried other op-amps based on a single value, like the slew rate. The AD8014 has a really high slew rate, but it didn't turn out well. For a single TIA with a single SiPM, it kind of 'double-peaked' somehow, so I don't know much.

duak:
I haven't worked with SiPMs and, until I saw your post, didn't know they existed.  I see that Digikey has some for $40.  If they're sensitive to some types of particle radiation, I might be tempted to get one to fiddle with.   Anyway, the last photodiode amplifier I designed was about 15 years ago so I'm a bit rusty on the details of TIAs.  It worked with a medium area photodiode and had a bandwidth of a MHz or so.  Hopefully, some of that knowledge is applicable to SiPMs.

The capacitor across the feedback resistor in a TIA helps compensate for the photodiode (or SiPM's) capacitance to ground and generally prevents the opamp from oscillating.  Its value depends on the feedback resistor, the various capacitances on the summing node and the gain vs frequency characteristics of the opamp.  I would think that its value should increase proportionaly to the number of SiPMs in parallel.

About the opamp output current:  looking at figure 6 on p.4 of the J-series SiPM data sheet I see a peak voltage of 70 mV into a 10R sense resistor.   This gives a 7 mA peak photocurrent that must be balanced out by the TIA.  I'm assuming the current pulse is from a photon triggering a single photodiode - AND9770 shows that higher currents are possible when multiple photodiodes are triggered.   The opamp can deliver at least +/- 48 mA, some of which can drive a 50 ohm load and the remainder to balance the photocurrent.  Are there any conditions that require the opamp to deliver more than 50 mA?

I found an interesting paper from CERN about SiPMs and readout electronics: https://indico.cern.ch/event/164917/contributions/1417117/attachments/198508/278657/1-cdlt_Photodet2012.pdf

A schematic with circuit values and operating conditions and some images of the waveforms you're having trouble with will help us understand what you're seeing.

Cheers,

jmelson:

--- Quote from: LoveLaika on June 18, 2020, 06:31:13 pm ---Now, it works fairly well with a single SiPM, so I tried to put more SiPMs in parallel to construct arrays. However, when you put SiPMs in parallel, you increase the input capacitance and you increase the noise from each one when you connect them all to one TIA, and one output.

--- End quote ---
Right, we've gone back and forth with this at work.  My boss thinks that multiple SiPMs will be great because they collect more light.  And, in theory, the light goes up by a factor of N, and the dark current pulses also go up by N, but add in quadrature.  But, the killer is the capacitance.  That ruins the performance of the amplifier.  We were really hot to make a large detector system with hundreds of SiPMs, and I got a preamp that worked well.  The Sensl (now ON Semi) SiPMs have the fast output, but the dark current is horrible.  We moved over to Hamamatsu devices, and the dark current is almost an order of magnitude less.  But, we still had issues with the discharge/reset of the micropixels causing a long tail.  Also, it seemed that the tail became longer with greater pulse amplitude, therefore changing the pulse shape.  Since we were interested in doing pulse shape discrimination, a constant pulse shape was pretty important.

Jon

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