1. 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

2. 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.

3. 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

4. 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.

5. 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

6. 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.

7. 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.

8. 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

9. 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.

10. 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.

11. 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.

12. 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

13. 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.

14. 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 >
Small signal linearity is much improved.
Dynamic range：10pC – 1000pC.
The non-uniformity for all the channels is within ±5.5% for the ASIC.

15. Timing Measurement 100ps – 300ps5 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.

16. Energy Measurement ASIC intrinsic: ~0.1% FWHM @ 500pC input.
With detector: ~10% 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 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.

17. 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