Front-end for Silicon Photomultiplier (SiPM) POLITECNICO DI BARI Front-end for Silicon Photomultiplier (SiPM)

SiPM: Silicon photomultiplier MATRICE DI FOTODIODI A VALANGA POLARIZZATI IN GEIGER MODE MOLTIPLICAZIONE DEI PORTATORI TRAMITE IL PROCESSO A VALANGA AMPIEZZA DELl’IMPULSO DI USCITA PROPORZIONALE AL NUMERO DI FOTONI ASSORBITI 24x24 pixels Politecnico di Bari

Cg  10pF, without considering the fringe parasitics Electrical model of a SiPM Rq: quenching resistor (hundreds of kW) Cd: photodiode capacitance (few tens of fF) Cq: parasitic capacitance in parallel to Rq (smaller than Cd) IAV: current source modelling the total charge delivered by a microcell during the avalanche  Cg : parasitic capacitance due to the routing of the bias voltage to the N microcells, realized with a metal grid. Example: metal-substrate unit area capacitance 0.03 fF/mm2 metal grid = 35% of the total detector area = 1mm2  Avalanche time constants much faster than those introduced by the circuit: IAV can be approximated as a short pulse containing the total amount of charge delivered by the firing microcell Q=DV(Cd+Cq), with DV=VBIAS-VBR Cg  10pF, without considering the fringe parasitics Politecnico di Bari

a) Transimpedance amplifier Experimental validation of the model Two different amplifiers have been used to read-out the FBK-irst SiPM a) Transimpedance amplifier BW=80MHz Rs=110W Gain=2.7kW b) Voltage amplifier BW=360MHz Rs=50W Gain=140 The model extracted according to the procedure described above has been used in the SPICE simulations The fitting between simulations and measurements is quite good Politecnico di Bari

Charge sensitive amplifier Front-end electronics: different approaches - + SiPM Vbias CF VOUT + - RS SiPM Vbias VOUT RS SiPM Vbias IS kIS=IOUT Charge sensitive amplifier Voltage amplifier Current amplifier The charge Q delivered by the detector is collected on CF If the maximum DVOUT is 3V and Q is 50pC (about 300 SiPM microcells), CF must be 16.7pF Perspective limitations in dynamic range, die area, power consumption A I-V conversion is realized by means of RS The value of RS affects the gain and the signal waveform VOUT must be integrated to extract the charge information: thus a further V-I conversion is needed RS is the (small) input impedance of the current buffer The output current can be easily replicated (by means of current mirrors) and further processed (e.g. integrated) The circuit is inherently fast Less problems of dynamic range Politecnico di Bari

The CMOS current buffer 0.35mm standard CMOS technology Common gate configuration (M1) Feedback applied to increase bandwidth and decrease input resistance (M3, M2) SiPM bias (and gain) fine tuning possible by varying Vrif Main simulated specs Small signal bandwidth: 250MHz  Input resistance: 17W Total current consumption: 800uA  Linearity dynamic range: about 50pC Rise time of the output waveform: 400ps  3.3V power supply Vrif variable in the range 1V÷2V Politecnico di Bari

Experimental setup: blue LED light source Iout 50Ω Pulse Generator Current Buffer Voltage Amplifier BNC SiPM RIV The circuit has been coupled to a SiPM realized by FBK-Irst 7V Picture of the setup Single dark pulse measurement (Vbr=-30.5V; Vbias=-32.5V) Politecnico di Bari

Dark pulse measurements Charge measurements at Vbias = -32.5V Comparison with a very fast discrete voltage amplifier front-end, used as a reference: Average dark pulse charge Integrated current buffer: 143fC Discrete voltage amplifier: 142fC The standard deviation is worse: sint2sdisc Blue LED measurements Comparison with the ref. amplifier : Average no. of fired microcells Current buffer: 39 Ref. amplifier: 38.4 Standard deviation Current buffer: 7.5 Ref. Amplifier: 7.2 Average number of fired microcells as a function of the input pulse width Charge distribution for a 8.25ns input pulse width (in terms of no. of fired microcells) Politecnico di Bari

Architecture of the analog channel Variable gain integrator: Gain: 1V/pC  0.33V/pC (2 bits); f = 200ns; Output voltage range: 0.3V ÷ 2.7V; Current mirror scaling factor 10:1 Current discriminator: Current mirror scaling factor 1:1; Threshold variable from 0 to 40µA (about 50 microcells @ VBIAS=-31.5V); Baseline holder : Baseline value Vbl = 300mV Very slow time constant; Non-linearities added to prevent baseline shifts at increasing event rates Politecnico di Bari

Typical output waveforms Experimental setup: LED light source Lemo BlueLed SiPM Vbias 50Ω Lemo Pulse Generator Voltage Buffer Logic Buffer Chip Ch_in Ch_out Disc SiPM A51 ( FBK – IRST ) Blue Led HSMB-C150 Typical output waveforms (Vbias=31.5V) Politecnico di Bari

Charge measurements (blue LED light source) Ouput voltage vs pulse width for different gain settings From the previous characterization measurements we have: For pulse width = 9ns, n=115 fired microcells  If Vbias = 31.5 V, the total injected charge is QT= Q µcell(31.5V)*n = 6.9pC If Vbias = 32.5 V, the total injected charge is QT= Q µcell(32.5V)*n = 17.3pC Vbias 31.5V 32.5V QT/(M*Cf) 690mV 1.73V Vpeak-Vbl 670mV 1.76V Measurement are in good agreement with the expected results Politecnico di Bari

Design of the 8 channel ASIC: the Peak Detector (PD) It is based on a P-MOS current mirror as a rectifying element IBIAS added to improve the speed of operation, especially for small signals OTA _ + Integrator output out IBIAS VDISC Chold=2pF reset VDD M1 M2 MR Politecnico di Bari

Design of the 8 channel ASIC: the fast-OR Fast-OR circuit operating in current mode, to improve the speed of operation Current buffer to reduce the input impedence Current discriminator with fixed treshold Cur_disc M0 M1 M7 trig_0 trig_1 trig_7 Vdd Vbias Ibias I0 I1 I2 Ithresh Cbus F_or Ithresh= I2-(I0-I1) MNBUF MPBUF Politecnico di Bari

Architecture of the test chip Politecnico di Bari config_reg DAC Vrif I_th MUX_reg Trig. reg. CSA Curbuf PD Curdisc MUX ADC Read_out logic srq_pad MUX_sel F_or reset_pad ck_pad rw_pad data_pad gain ch_0 ch_1 ch_7 EOC ADC_ck a_out_0 a_out_1 a_out_7 trig_0 trig_1 trig_7 data Ext. bias

Design of the 8 channel ASIC: Layout Politecnico di Bari

Read-out procedure for the test chip Package SMD A) An event activates the SRQ bus (by default at Hi-Z) B) FPGA gives a time-stamp to the event and takes control of the SRQ bus during the read-out procedure C) SRQ, in its active state, is used to “freeze” the content of the trigger registers (no more trigger are accepted) D) FPGA waits the time needed by the PDs to reach the peak and sends the CLOCK signal to the ASICs F) The read-out logic starts the A/D conversions and sends the results to FPGA on the DATA_i pad G) When all the conversions have been completed, FPGA releases the SRQ bus and sends a RESET signal SRQ CHIP DATA CLOCK FPGA DATA_0 RESET Politecnico di Bari

Valore medio del ritardo Valore medio del ritardo Jitter measurements on fast-OR signal Misure di jitter in presenza di un solo canale soprasoglia Misure di jitter in presenza di due canali soprasoglia Canale colpito Valore medio del ritardo Deviazione standard 1 1.77 ns 50 ps 2 1.66 ns 53 ps 3 1.68 ns 49 ps 4 1.69 ns 5 1.74 ns 48 ps 6 1.57 ns 7 8 1.67 ns Coppia canali Valore medio del ritardo Deviazione standard 1-5 1.618 ns 47.26 ps 2-4 1.55 ns 50.5 ps 2-3 48.6 ps 3-4 1.57 ns 49.7 ps 1-4 1.62 ns 49 ps 4-8 1.53 ns 50.3 ps 3-7 50 ps 6-8 1.415 ns 7-8 Politecnico di Bari

Design of the 32 channel ASIC: Logic Readout pd_out_0 Vrif I_th ex_ADC gain DAC Vrif DAC I_th pd_out_1 ADC_2 MUX DEMUX data MUX config_reg ADC_1 pd_out_31 DEMUX_reg reset_pad MUX_reg rw_pad MUX_reg Logic Readout EOC ADC/Clock Manager cK_ADC coincidence_pad ck_pad trig_0 Trig. reg. trig_1 SDI_pad F_or srq_pad SDO_pad trig_31 SS SPI interface Politecnico di Bari