X. Zhu1, 3, Z. Deng1, 3, A. Lan2, X. Sun2, Y. Liu1, 3, Y. Shao2 TIMPIC-II: the second version time-based-readout ASIC for SSPM Based PET applications X. Zhu1, 3, Z. Deng1, 3, A. Lan2, X. Sun2, Y. Liu1, 3, Y. Shao2 Dept. of Engineering Physics, Tsinghua University, Beijing, China Dept. of Imaging Physics, University of Texas, M.D. Anderson Cancer Center, Houston, Texas, USA Key Laboratory of Particle & Radiation Imaging, Ministry of Education, Beijing, China backup： A second version ASIC for front-end detector readout, TIMPIC-II, with higher density, simplified readout and better performance, has been developed for Solid-State Photomultiplier (SSPM) based PET applications. 核探测与核电子学国家重点实验室——清华大学工程物理系ASIC讨论会 Thursday 30 May 2013, Beijing, China Email: zhuxz10@mails.tsinghua.edu.cn

SSPM(SiPM/MPPC) Detector Features： Main Requirements for Electronics： Vbias Iout Detector Features： 106 gain fast rising time Compactness Immunity to magnetic field Main Requirements for Electronics： Wide input range High band-width High density readout electronics Solid state Photo-Multiplier detectors, which is also known as SiPM or MPPC, are gaining popularity in nuclear science and medical imaging because of its features such as large gain, fast signal rising time. the main requirements front-end circuit are, the wide input range, the high bandwidth and high density.

Time Based Readout Features of Time Based Readout: Current Discriminator T Input current pulse IN OUT Current Buffer TDC Integration and Discharging Replicating input pulse Charge-to-time Converter IN The Time Based Readout is adopted here, the input current pulse is replicated by a current buffer. One of the copy is sent to current discriminator for timing, and the other one is integrated and discharged to get the energy information, the output digital pulses of ASIC will be processed by TDC. Since the Time based readout is a current mode circuit approach, high band-width and wide input range can be easily achieved with appropriate power consumption. OUT Q Features of Time Based Readout: Current mode circuit approach High band-width Wide input range

TIMPIC-I VS TIMPIC-II TIMPIC2 Spec Channel Pin Pout Input direction Input range Control mode TIMPIC1 8chs 8 16 Negative 1000pC Single TIMPIC2 16chs Positive 200pC & 800pC Multiple TIMPIC2 Spec Dual Input Range 200pC & 800pC Input channel number 16 Noise charge (RMS) < 250fC Output channel number Bandwidth >100MHz Amplitude output number 1 Input Impedance ~50ohm 16-channel current SUM Timing resolution ~1ns (FWHM) Common trigger Power Consumption <10mW/channel adjustable range of discharge current 6 to 50µA Major differences between TIMPIC2 and its last version TIMPIC1 and the design specification are listed here. Channel number is double from 8 to 16, and signal polarity is changed to match most SSPM vendors’ configuration Dual input signal range (x1 or x4) selection allow TIMPIC2 to be used in single pixel mode and light sharing mode.

Architecture Integration & discharging Common Trigger This is the brief architecture of the ASIC. The input current is mirrored to three branches with different ratio, two of them are processed by a leading edge (LE) current discriminator and charge-to-time converter to measure the signal timing and energy. the other copy is added up together with all the other channels’ copies of input current to generate a common trigger signal, which will be discussed soon Common Trigger

Major Improvements in Circuit Design Threshold of each channel can be lowered Improve spatial & timing resolution Common Trigger Optimize the signal-to-noise ratio Enable single output pin per channel Constant Width Integrator For different applications Multiple Operating Modes several major changes have been made to make TIMPIC-II more flexible and suited for PET applications, including： The common trigger Constant width integrator Multiple Operating modes （constant means the integration time doesn’t vary with different input, but the integration time can be adjusted externally) Backup: The design introduces externally adjustable fixed charge integration time and each digital output encodes both interaction timing and energy information. Besides the pedestal threshold of each input channel, a common threshold for all inputs summing signal is added for improved event triggering of multiple interactions performance.

Common Trigger TIMPIC-I TIMPIC-II Select the true events, and Ignore the LYSO intrinsic low energy background. Channels act when there is a common trigger. Threshold of each channel can be lowered. Improving spatial and timing resolutions. Threshold Summing Current of 16 Channels Th_sum Common Trigger ch1 ch2 ch5 ch1 ch2 ch5 Each channel’s firing indicates an event High threshold is needed to prevent mis-trigger events. In Timpic1, Each channel’s firing indicates an event, so a high enough threshold is needed to prevent the mis-trigger events. For example, in light sharing mode, an event triggers 3 channels at the same time, but the small signal in channel 5 is lost because of the high threshold, that will reduce the timing and energy resolution. In TIMPIC2, A common trigger is introduced. a high threshold can be set to the summing current of 16 channels to select only the true events and ignore the scintillator’s intrinsic low energy background, channel only acts when there is a common trigger, so threshold of each channel can be lowered to improve spatial and timing resolutions. Backups: Chip control logic allows users to select the recorded event data from the common trigger or individual channels. This is helpful for identifying and filtering out the scintillation events for PET application.

Constant Width Integrator TIMPIC-I TIMPIC-II Gate width can be adjusted to optimize the signal-to-noise ratio. Enable single output pin per channel Ith Ith Ith-Ihys Ith Ith Ith-Ihys Integration time is the time over threshold Small signals have short integration time and bad linearity In timpic-1, integration time is the time over threshold, small signals have short integration time and bad linearity. In timpic-2, The constant integration time is used, so the linearity of small signals is improved and the integration time width can be adjusted externally to optimize the signal-to-noise ratio. And constant integration time also allows us to use single output pin per channel, since the leading edge of the output is the timing, and the width of output minus this constant integration time width is the discharging time, which means the energy. Integration & discharging Output Constant Discharging time

Operating Modes Free running Common trigger Synchronous discharging Single-pixel mode For channel calibration Light sharing mode Select true events Improve resolution Jitter of constant integration time Better energy resolution There are several operating modes we can choose. firstly, in the free running mode, Each channel integrates and discharges separately, and common trigger is invalid. This mode is for single-pixel application, and we use it for channel calibration.

Operating Modes Free running Common trigger Synchronous discharging No common trigger Free running Common trigger Synchronous discharging Single-pixel mode For channel calibration Light sharing mode Select true events Improve resolution Jitter of constant integration time Better energy resolution The common trigger mode has been discussed before, it is useful for light sharing mode to select true events and could improve timing and energy resolution. If one channel is mis-triggered with out a common trigger signal, it will be fast reset after 50ns.

Operating Modes Free running Common trigger Synchronous discharging Single-pixel mode For channel calibration Light sharing mode Select true events Improve resolution Jitter of constant integration time Better energy resolution In synchronous discharging mode, all the 16 channels start discharging at the same time at the falling edge of the common trigger. Energy is measured by the common trigger falling edge to the individual channel output falling edge. If the jitter of the constant integration time is high, better energy resolution can be achieved in this mode.

Chip Layout & Packaging ASIC layout ASIC in a 121 pins BGA package The die area is 3mm x 3mm，The ASIC chip is based on 0.35um technology and with BGA package. Layout of one channel

Chip Test Power consumption: ~1mW/channel. ASIC evaluation board AISC output signal capture from Oscilloscope This ASIC is tested and used by people from MD Anderson cancer center, they have a poster presentation focus on evaluation of TIMPIC2. Some primary test results will be shown here. low power consumption ~1mW/channel. Power consumption: ~1mW/channel.

Linearity & Uniformity Uniformity of 16 channels in one chip Linearity is good and small signal linearity is much improved, the dynamic range is 10pC-1000pC。 The non-uniformity of 16 channels in one ASIC is about 10% Linearity response with R > 0.999. Small signal linearity is much improved. Dynamic range：10pC – 1000pC. The non-uniformity for all the channels is within ±5.5% for the ASIC.

Timing Measurement 100ps – 300ps 5 10 15 20 25 30 35 40 200 400 600 800 1000 Time walk Leading edge Timing (ns) input charge (pC) 0.05 0.1 0.15 0.2 0.25 0.3 200 400 600 800 1000 Timing jitter for different input pulses Timing jitter - standard deviation (ns) input charge (pC) 100ps – 300ps Time walk and time jitter for different input charge are measured with pulses that have the same shape as the detector output pulses. The timing jitter standard deviation is 100ps – 300ps for different input signal amplitude. Measured with pulses that have the same rise and fall time constants as the detector input pulses but with different amplitudes.

Energy Measurement ASIC intrinsic: ~0.1% FWHM @ 500pC input. With detector: ~10% FWHM @ 500pC input.(Sensl SSPM at 30.2V bias voltage) Maximum output pulse width(dead time) can be adjusted from 400ns to 10us by changing the discharge current. Energy spectrum Energy spectrum from Na-22 source. The detector is Hamamastu MPPC array and the crystal is LYSO. The FWHM at 511keV is about 15%-16%. Temperature drift Within 1% from 29℃ to 70℃ The ASIC’s intrinsic energy resolution is about… The energy resolution tested with sensl’s sspm biased at 30.2V is 10% @ 500pC. Here is an energy spectrum measured with sodium 22. MPPC, LYSO, 511keV The maximum output pulse width… The temperature drift of energy is within 1% from 29 to 70 centigrade. 全能峰：full-energy peak Na：sodium 摄氏度：centigrade。 Full width at half maximum.

Summary TIMPIC-II is a Successful upgrade: Better linearity & resolution Higher density Dual input range(Dynamic range: 10pC – 1000pC) Multiple operating modes A practical electronics system with TIMPIC-II was developed 核探测与核电子学国家重点实验室——清华大学工程物理系ASIC讨论会 Thursday 30 May 2013, Beijing, China Email: zhuxz10@mails.tsinghua.edu.cn