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Multiplexing Approaches for a 12 x 4 Array ofSilicon PhotomultipliersChen-Yi Liu and Andrew L. Goertzen, Member, IEEE

Abstract—Two resistor network multiplexing circuits for a 124 array of SiPMs were constructed and tested. Both circuits en-code the position and energy information from 48 SiPM pixels inonly 4 analog channels. The two circuits differ in that one bufferseach SiPM output with a non-inverting voltage-feedback opera-tional amplifier before multiplexing, whereas the second one con-nects the output of the SiPMs directly to a charge division resistornetwork. The energy and timing resolution were measured with a4 4 array of LYSO scintillator crystals with size matched to theSiPM pixel size. The measurement was done in 3 steps to cover all12 4 SiPMs. Both circuits gave an energy resolution of 14%. Thesingle sided timing resolution for the buffered output circuit was2.86 ns, using a 350–650 keV energy window. In comparison, thetiming for the circuit with direct connections between SiPMs andthe resistor network was 3.54 ns, using the same energy window.Based on these results, the predicted coincidence timing resolutionsare 4.0 ns and 5.0 ns, respectively. The coupling of the SiPM capac-itance with the resistor network results in different signal shapingtime constants for different SiPMs in the passive resistor network,causing a delay in trigger time for the inner SiPM signals. On theother hand, the circuit with buffer amplifiers does not suffer fromthis effect, and the pulse shape is more uniform across the SiPMs.We also demonstrate, using a 10 10 array of 1.5 mm LYSO crys-tals, that the inclusion of multiple SiPMs in both circuits reducesthe detector’s ability to resolve crystals in the flood histograms.The amount of noise increases with number of SiPMs in the mul-tiplexing circuit.

Index Terms—Positron emission tomography, signal multi-plexing, silicon photomultiplier.

I. INTRODUCTION

S ILICON photomultipliers (SiPMs) have been extensivelystudied as detectors for Positron Emission Tomography

(PET) imaging systems. Because the size of an individual SiPMis usually small (i.e. or less), many designs com-bine arrays of SiPMs to make detector modules with larger sur-face areas [1]–[3]. Often the signals from these SiPMs are mul-tiplexed to reduce the number of data lines leaving each module[4]–[6]. The reduced number of data lines means fewer analog

Manuscript received February 19, 2013; revised June 27, 2013 and September12, 2013; accepted September 23, 2013. This work was supported in part by theNatural Sciences and Engineering Research Council of Canada under a Dis-covery Grant to A. L. Goertzen and by the Manitoba Health Research Councilunder a Studentship Award to C.-Y. Liu.C.-Y. Liu is with the Department of Physics and Astronomy, University of

Manitoba, Winnipeg, MB, Canada.A. L. Goertzen is with the Department of Radiology and the Department

of Physics and Astronomy, University of Manitoba, Winnipeg, MB, Canada(e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TNS.2013.2283872

channels to be digitized in the downstream electronics are re-quired, thus lowering the cost and complexity of the data acqui-sition electronics required for the system.Many approaches to multiplexing SiPM signals have been

studied. For example, Wang et al. [1] employed a row/columnsumming method that was first developed for multi-anode pho-tomultiplier tubes [7]. In this method, the signals from SiPMsresiding in the same row/column are combined. These combinedsignals are then used to derive the values,from which the position and energy of the scintillation eventcan be calculated. In another approach described by Song et al.[8], as well as by others [4], [9]–[11], the SiPM signals are fedinto a charge division resistor network inspired by the propor-tional counter, and hence named DPC (discretized proportionalcounter) circuit. This multiplexing strategy was originally de-veloped for multi-anode photomultiplier tubes as well [12] buthas been adapted to be used on SiPMs. Like the row/columnmultiplexing method, the DPC circuit also produces 4 outputs,one at each corner of the square circuit, and the position and en-ergy can be calculated from these four signals.Several studies have applied these multiplexing techniques

to an array of 4 4 SiPMs, which is usually a detector modulewith an area less than [3], [4]. A 4 4 array isused because packages of this size are readily available frommultiple vendors. Recently however, there have been reports onmultiplexing 8 8 and 12 12 arrays of SiPMs with good en-ergy resolution and ability to resolve small crystals [1], [2], [13].A detector with a larger area means that the entire PET systemcan be made up of fewer detector modules and fewer analogchannels. Our group is investigating multiplexing techniques asa way to extend the axial coverage of our PET insert designedto fit within a small animal magnetic resonance imaging (MRI)system [14], [15].This study investigates the performance of a 12 4 array

of SiPMs multiplexed with a DPC circuit. We tested and com-pared two versions of the circuit. The first version has the SiPMoutput passively fed into the resistor network, similar to pre-vious studies. The second version uses a DPC circuit that incor-porates an operational-amplifier at each SiPM output to convertthe current to a voltage signal before multiplexing. This paperalso examines the reduced crystal resolvability caused by theadditive noise of multiple SiPMs.

II. MATERIALS AND METHODS

A. SiPMThe SiPMs used in this work were the SensL SPMArray4

(SensL Inc., Cork, Ireland). The SPMArray4 is a package con-taining a 4 4 array of SiPMs, with each SiPM being

0018-9499 © 2013 IEEE


2 IEEE TRANSACTIONS ON NUCLEAR SCIENCE

Fig. 1. Charge division resistor network that passively multiplexes the 12 4SiPM signals to 4 channels. The #-#m connectors are the input points for theSiPM output. The SUM A/B/C/D contain the multiplexed signals.

in size and containing 3640 cells. The gain of eachcell is , and the dark cell firing rate of the SiPM is 8MHz.Tiling three SPMArray4 side by side creates a 12 4 array ofSiPMs with a total area of . We apply a 30.2 V biasvoltage to the SiPMs as suggested by the data sheet.

B. Passive Multiplexing Circuit

The multiplexing circuit is based on the design described in[11] and reduces the number of data channels from 48 to 4. The

Fig. 2. Multiplexed signals in the passive multiplexer are amplified with anOPA2690 op-amp configured as in this diagram. The signal gain is 13.5.

Fig. 3. Configuration of the buffer amplifier for the SiPM in the 1st row andthe 1st column. The configuration for other SiPMs is the same except for thosein the 2nd and the 3rd columns, where the output resistor is 250 instead of330 . The OPA4820 was chosen for its dense quad amplifier IC packaging.

passive multiplexing circuit connects the output of each SiPMto a resistive charge division network (Fig. 1). The four outputsare then amplified by Texas Instruments OPA2690ID (Texas In-struments, Dallas, TX) configured as inverting op-amps (Fig. 2).The signals leave the multiplexing board through LEMO 00connectors and 50 coaxial cables.

C. Active Multiplexing Circuit

The active multiplexing circuit has an additional non-in-verting voltage feedback op-amp (OPA4820, Texas Instru-ments, Dallas, TX) at the output of each SiPM and before theinput to the resistor network. Fig. 3 shows the configuration ofthe op-amp, which is similar to a design described in [9] and[16]. The output resistor of the buffer amplifier changes theamplitude of the summed signal, with a smaller resistor givinglarger amplitude. The resistor for columns 2 and 3 are loweredto increase the summed signal from these SiPMs in order togive a uniform response across all columns of the array.The resistor values in the network (Fig. 4) are also modified

to accommodate the new signal source from buffer amplifiersand to create a greater inter-row distance in the flood histogramamong the first four and the last four rows of SiPMs. Eventhough this introduces a physical distortion in the flood image, itmakes resolving crystals in the top and bottom areas easier. Thefour multiplexed outputs are further amplified by AD8132 dif-ferential amplifiers (Analog Devices, Norwood, MA) and thensent out via a High-Definition Multimedia Interface (HDMI)cable (Fig. 5) [17]. An adaptor board connects the negative endof the differential HDMI signal lines to LEMO 00 connectors. In


LIU AND GOERTZEN: MULTIPLEXING APPROACHES FOR A 12 X 4 ARRAY OF SILICON PHOTOMULTIPLIERS 3

Fig. 4. Resistor network in the active multiplexing circuit.

order to provide better consistency with the single ended trans-mission used in the passive multiplexor, a differential receiveramplifier was not used.

D. Flood Histogram and Energy Resolution

A 4 4 LYSO scintillator array with crystal size matched tothe SiPM pixel size (Proteus Inc., Chagrin Falls, OH) was cou-pled with optical grease to one of the three SensL SPMArray4s.The size of the crystal element is . Thearray was made with crystals polished on all sides and havinga bonded 3 M enhanced specular reflector (ESR) between crys-tals. There was only one 4 4 scintillator array available fortesting and thus the other two SPMArray4s were left withoutcrystals but were still connected to the circuit. After data col-lection at the first position, the scintillator coupled SPMArray4was unplugged and swapped position with another SPMArray4.

Fig. 5. In the active multiplexer, the multiplexed signal is amplified withAD8132 configured as shown here.

Fig. 6. (a) Setup of the NIM electronics for acquiring flood histograms andmeasuring energy resolution. (b) The setup for measuring single detector timingresolution.

Then, another set of data was collected. After rotating the crystalcoupled SPMArray4 through the three sockets, the three listmode data sets were combined and analyzed as if they were asingle measurement. A rod source was used to flood irra-diate the crystal array.During data acquisition, the four outputs of the multiplexing

circuits are sent to fast amplifiers (Phillips Scientific 778) to spliteach signal into two paths, one to a fast summing amplifier andone to a slow spectroscopy amplifier (Fig. 6(a)). The summingpath consists of a fan-in amplifier (Phillips Scientific 740) tosum the four signals for input to a constant fraction discrimi-nator (CFD) (Phillips Scientific 715). The delay line used for theCFDwas 30 ns. In the spectroscopy path, pulses are shaped witha 0.25 time constant using a spectroscopy amplifier (MesytecMSCF-16). The shaped signals are sampled and digitized at thepeak of the pulse using a custom built sample-and-hold and aNational Instruments PCI-6133 analog to digital converter card(National Instruments, Austin, TX).



Once the signals are digitized, the position of the event andits energy are calculated as:

(1)

(2)

(3)

The events are sorted into a flood histogram and segmented withcustom software in Matlab (Mathworks Inc., Natick, MA) togive energy and timing spectra on a per-crystal basis.

E. Single Detector Timing ResolutionThe single sided timing resolution was measured using a

Scanwell Systems Timing Probe (Scanwell Systems, Montréal,QC), which has a source embedded in a plastic scintillatorthat is coupled to a photomultiplier tube [18]. The timing reso-lution of the probe is 300–400 ps according to the informationsupplied by the vendor. Since the timing resolution of thecircuits examined in this present work is on the order of 2–4 ns,the timing results were not corrected for the timing resolutionof the timing probe. As shown in Fig. 6(b), The output of thephotomultiplier tube is fed directly into a CFD which uses a0.5 ns delay line, and the resulting trigger is used as the startsignal for a time-to-amplitude converter and single channelanalyzer (TAC/SCA) (Ortec 467 or Ortec 567, Ortec AdvancedMeasurement Technology, Oak Ridge, TN). The stop signal isthe CFD trigger from the summed SiPM signal.Due to the long internal delay in the time-to-amplitude

converter, the timing pulse arrives at the sample and holdmuch later than the four energy pulses. In order to sample allfive pulses simultaneously, the time constant of the MesytecMSCF-16 shaping amplifier was increased from 0.25 toeither 0.5 or 1 for the timing measurements. The same 44 LYSO scintillator array was also used for timing measure-

ment.

F. Effect of Number of SiPMs in the Multiplexing Circuit onCrystal ResolvabilityA concern when multiplexing large numbers of SiPM pixels

together is the additive effect of the dark count noise. In orderto investigate this problem, data were acquired with the multi-plexing circuits containing different numbers of SiPM arrays.We started by plugging in only one SPMArray4 to the mul-tiplexing circuit and acquiring flood histogram data. Then theother two SPMArray4 detectors were added one at a time tosee if the flood histogram changed as a result of the presence ofthese additional SiPMs. In this test, we used a 10 10 LYSOscintillator array with an element size of .The 10 10 crystal array was chosen instead of the 4 4 arraybecause the change in flood histogram will be more noticeablewith smaller crystals. The size of the 10 10 scintillator arraycovers an area of 15 mm 15 mm, so crystals on the edge ofthe array fall outside the sensitive area of the SiPM. As a result,only an 8 8 array of crystal blobs will show up clearly in theflood histogram. The 10 10 array was obtained from the samesupplier as the 4 4 array (Proteus, Inc.) and also had polishedcrystals and a bonded ESR reflector.

Fig. 7. Screenshot from an oscilloscope showing the sum of four output chan-nels from the (a) passive multiplexer and (b) active multiplexer.

III. RESULTS

A. Pulses for Timing Measurement

Fig. 7 shows the screen captures from an oscilloscope dis-playing the pulses coming out of the fast summing amplifier.These are the pulses that go into the CFD for event triggering.The time required for the pulse to rise from 20% to 80% of itsamplitude is approximately 40 ns for the passive multiplexerand 20 ns for the active multiplexer.

B. Flood Histogram

Fig. 8 shows the flood histograms for the two multiplexingcircuits investigated. All crystals in both flood histograms arewell resolved, indicating that it is possible to decode 48 SiPMsusing just 4 signals.Fig. 9 shows profile plots taken along the blue line in the flood

histograms. A functionmade of 12Gaussian curves was fit to theprofile to calculate the crystal resolvability index. The crystalresolvability index is defined as the average full-width-at-half-maximum divided by the peak-to-peak separation [19]. There-fore, a smaller value means better crystal resolvability.The crystal resolvability index of the passive multiplexer is

0.31 0.12, whereas the resolvability index for the active mul-tiplexer is 0.25 0.05. The smaller value in the active multi-plexer means it is better of the two at resolving small crystals.



Fig. 8. Flood histograms obtained with the (a) passive multiplexer and (b) ac-tive multiplexer. Only events with energy higher than 250 keV are includedhere. The active multiplexer employs a different resistor network that changesthe location of the crystal blobs in the flood histogram. The blue line along thesecond column indicates the path of the profile in Fig. 9.

The larger separation in the first and last four peaks in the activemultiplexer also facilitates the identification of crystals.

C. Energy Resolution

Fig. 10 shows example of the energy spectrum from both cir-cuits and Fig. 11 shows the photopeak position and energy reso-lution of the 511 keV photopeak measured by the passive multi-plexer. The photopeak position is quite uniform across the arraywith an average position of 3.1 0.2 V. The energy resolutionis also uniform across the array with an average value of 14.20.4%.Fig. 12 shows the photopeak position and the energy resolu-

tion of the 511 keV photon measured by the active multiplexer.

Fig. 9. Profile plots of the flood histograms in Fig. 8. (a) The passive mul-tiplexer and (b) active multiplexer. The blue line is the profile, and the blackdotted line is the Gaussian fit to the peaks.

Fig. 10. Example energy spectrum of 511 keV photons measured with one ofthe crystals in the (left) passive multiplexer and (right) active multiplexer. Thecrystal of choice is the one in the fifth row and the second column. The blueline is the actual spectrum, and the black curve is a Gaussian and linear fit ofthe photopeak. The photopeak position and energy resolution are derived fromthis fitted curve.

The average photopeak position is 2.9 0.3 V, and the averageenergy resolution is 13.9 0.5%. The central SPMArray4 hasa higher photopeak amplitude than the other two. This can becorrected by using resistors slightly higher than 330 and 250at the output of the central SPMArray4’s buffer amplifiers.

D. Single Detector Timing Resolution

Fig. 13 shows the trigger time and timing resolution of eachcrystal in the passivemultiplexing circuit. The individual crystaltiming resolution is on average 3.5 0.2 ns for events withinthe energy window 350–650 keV. Even though there is a 3–4



Fig. 11. (a) Plot of the photopeak position of 511 keV photon in the energyspectrum measured by the crystals in the passive multiplexer. (b) Plot of theenergy resolution at 511 keV.

Fig. 12. (a) Plot of the photopeak position in the energy spectrum obtained bythe active multiplexer. (b) Plot of the energy resolution at 511 keV.

ns difference in trigger time between different crystals, this dif-ference can be corrected in software by applying a time align-ment on a per crystal basis. The block timing performance after

Fig. 13. (a) Trigger time delay of each crystal in the passive multiplexer. Thevalues are relative to the crystal that has the earliest arrival time, which is set tozero. (b) Timing resolution calculated for each crystal.

Fig. 14. Block timing resolution of the passive multiplexer after timingalignment correction applied on a per-crystal basis. The black dotted curve is aGaussian fit to the experimental data shown in blue. The resolution is definedas the full width at half maximum of this Gaussian curve.

timing alignment correction is 3.54 ns, as shown in Fig. 14. Fora wider energy window of 150–750 keV the timing resolutionis 4.6 0.3 ns on a per crystal basis and 4.58 ns as a block.Fig. 15 shows the trigger time and timing resolution for each

crystal in the active multiplexing circuit. Unlike in the passivemultiplexer, the trigger time does not show a systematic delayfor crystals in the middle. The average timing resolution is 2.70.2 ns for events within the 350–650 keV energy window.

Fig. 16 shows the 2.86 ns block timing resolution after timealignment correction. Widening the energy window to 150–750keV degrades the timing resolution to an average of 3.3 0.2 ns



Fig. 15. (a) Trigger time delay of each crystal in the active multiplexer. Thevalues are relative to the crystal that has the earliest arrival time. (b) Timingresolution for the individual crystals.

Fig. 16. Block timing resolution of the active multiplexer.

per crystal and 3.41 ns as a block, a relatively small degradationcompared to the passive multiplexing circuit.

E. Effect of Number of SiPMs in the Multiplexing Circuit onCrystal Resolvability

We observed that adding more SiPMs to the multiplexing cir-cuit deteriorated the flood image, as shown in Fig. 17. Table Ilists the resolvability index of the two detectors reading 1.5mm crystals when different numbers of SiPMs are included inthe circuit. The resolvability index is calculated from a profiledrawn across the fourth column of crystals in the flood his-togram. The 1.5 mm crystals cannot be resolved when there are12 4 SiPMs in the circuit.

This blurring effect is likely due to the added dark currentnoise from the additional SiPMs. The noise decreases the posi-tion accuracy primarily in the tiling direction, that is, the verticaldirection in the flood images shown here.

IV. DISCUSSIONIn this work, the active multiplexer demonstrated better

crystal resolvability than the passive multiplexer. This result islikely due to the modification of the resistor network values toincrease crystal blob spacing in the flood histogram. This mod-ification can be readily done with the active multiplexer dueto the voltage outputs of the buffer amplifiers but not readilydone with the unbuffered outputs of the passive multiplexingcircuits. Because the data were collected over three steps,there are no inter-crystal scatter events between the 4th and5th rows, and between the 8th and 9th rows of crystals in theflood histogram and in the profile plot. If a continuous array ofscintillator crystals is mounted on the SiPM array, a light guideis required to readout crystals that sit above the insensitive areaof the SiPM array packaging.The passive and active multiplexers have similar energy res-

olution, an average of 14% for both circuits. This is comparableto the 12%–16% range reported by Majewski et al. using theirrow/column multiplexing strategy with 9 of the same SensLSiPM arrays [2].The timing performance of our passive multiplexing circuit

is limited. The slow pulse rise time limits the timing accuracyand is presumably due to the limited bandwidth of the amplifierand also the coupling of the SiPM capacitance to the resistor net-work, which forms an RC low pass filter. In addition, the SiPMsexhibit two categories of trigger time. The pulses from SiPMsthat reside in the inner two columns trigger approximately 3–4ns later than those from SiPMs that sit in the left and right mostcolumns (Fig. 13(a)). The delay in the inner columns is likelydue to the extra resistors and the intrinsic SiPM capacitancethat these signal pulses need to go through. Without a timingalignment correction, this separation in trigger time resulted ina block timing resolution of 6.63 ns. With a per crystal delaycorrection, the block timing resolution reduces to 3.54, muchcloser to the 3.5 0.2 ns per crystal timing resolution.The active multiplexing circuit, in contrast, does not have the

issue of SiPM capacitance coupling to the resistor network. TheSiPMs in the inner two columns trigger at about the same timeas SiPMs in the outer columns. However, the SiPMs at one endof the array trigger 1–2 ns later than the SiPMs at the otherend. This difference may be caused by the stray capacitanceof the signal traces or their uneven lengths on the printed cir-cuit board as the extra op-amps and the associated componentsmade routing the signal paths a challenge in a confined space.Routing traces in the passive multiplexer was a much simplerproblem because of the smaller number of components on theboard, making the trace length difference easier to minimize.Themultiplexed signals from the active multiplexer also have

a shorter rise time as shown in Fig. 7. This may be due tothe higher bandwidth of the amplifier configuration of the cir-cuit. Consequently, the timing resolution improved. For an en-ergy window of 350–650 keV, the timing resolution is 2.86 ns,slightly better than the 3.54 ns for the passive multiplexer.



Fig. 17. Adding SiPMs to the multiplexing circuit introduces noise that de-grades the quality of flood histogram. The flood histograms shown here are ob-tained with the active multiplexer. These flood histograms show only events thatare above 350 keV. The blue line indicates the profile that is used for calculatingthe resolvability index.

TABLE IRESOLVABILITY INDEX OF THE DETECTOR WHEN DIFFERENT NUMBERS OF

SiPMs ARE BEING MULTIPLEXED

The values reported here are all single sided timing resolu-tions measured in coincidence with a fast plastic scintillator de-tector. Multiplying these values by gives the theoretical co-incidence timing resolution of the detector, which turns out tobe 5.0 ns for the passive and 4.0 ns for the active multiplexer.The timing performance of the timing probe is not consideredin this calculation because of its relatively small contribution,300–400 ps.

The 4.0 ns coincidence timing resolution predicted of the ac-tive multiplexer is worse than the 2.6 ns coincidence timing res-olution reported by Wang et al. who used the row/column mul-tiplexing strategy with 4 SensL SiPM arrays [1]. However, theyderive their timing signal from the SiPM bias line rather than themultiplexed signals, which may be the reason for their better re-sult.Kolb et al. and Song et al. both used the summed multiplexed

signals for timing measurement as in this study, and they re-ported a 1 ns timing resolution using 9 single pixel HamamatsuMPPCs [8], [9]. Because the multiplexing approach they use issimilar to our passive multiplexer, the better result they have islikely due to the better timing performance of the Hamamatsudevices compared to the SensL SPMArray4 used in this work.Timing measurements done in our lab using a single SiPM pixelof the SensL SPMArray4 gave a timing resolution of 1.6 nsFWHM, whereas using the Hamamatsu MPPC gave a timingresolution of 800 ps [20]. Both tests were done in coincidencewith the same Scanwell timing probe. The 1.6 ns single pixeltiming resolution also suggests that multiplexing degrades theoverall timing by 1–2 ns.A few changes in our experimental setup may improve the

circuit timing performance. One is using a higher bandwidthop-amp or a common base amplifier as described by Huizengaet al. [16] to ensure the amplifiers do not increase the rise timeof the SiPM signal pulse. Even though high bandwidth voltage-feedback op-amps were chosen at the time of our circuit design,their 300–350 Mhz gain-bandwidth products may not be largeenough for our multiplexers. In the case of the passive multi-plexer where the gain is 13.5, the bandwidth of the op-ampsis around 24 MHz, which will attenuate the high frequencycomponent of the signal pulse. As Wang shows in [21], timingperformance depends on the high frequency component of theSiPM signal. The bandwidth limitation of the op-amps was notexamined in this study. Secondly, we can investigate using awaveform sampler instead of a CFD to determine the pulse ar-rival time more accurately, possibly leading to improvements inthe timing resolution measured.Lastly, we demonstrated that adding SiPMs to the multi-

plexing circuit reduces our ability to resolve crystals in theflood histogram. The noise from SiPMs will be a limitingfactor on the maximum number of SiPMs we can include in themultiplexing circuit.

V. CONCLUSION

Two versions of a SiPMmultiplexing circuit, one passive andone active, were built and tested. Both circuits can be used for a12 4 array of SiPMs. The energy resolution of LYSO coupledto SiPMs was 14.2 0.4% for the passive and 13.9 0.5%for the active multiplexer. The single sided timing resolution ofthe passive circuit was 3.54 ns while for the active multiplexer, abetter timing resolution of 2.86 ns was measured, with estimatedcoincidence timing resolution of 5.0 ns and 4.0 ns respectively.It was found that multiplexing too many SiPMs also has its dis-advantages as the extra noise coming out of the SiPMs reducesthe ability to resolve crystals in the flood histogram.



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