Comparison Between DRS4 Chip-Based Boards and ADCs for a Flexible PET Electronics

Transcription

1 Comparison Between DRS4 Chip-Based Boards and ADCs for a Flexible PET Electronics D. Stricker-Shaver 1, S. Ritt 2, B. Pichler 1 1 Laboratory for Preclinical Imaging and Imaging Technology of the Werner Siemens Foundation Department of Preclinical Imaging and Radiopharmacy, University of Tuebingen 2 Paul Scherrer Institute(PSI), Villigen, Switzerland Department of Preclinical Imaging and Radiopharmacy

2 Motivation Future Time-of-Flight (TOF) PET Systems ideally should function with an CRT < 400ps (FWHM) Get time information from fast TDC, analog timing pickoff Digitize Waveforms with a fast (GS/s) ADC, digital timing pickoff Power consumption will become a big issue DRS4-chip looks like a promising alternative (6GS/s) The chip could not compete with ADCs-> right Calibration is missing

3 Measuring Time Differences Function Generator Delay Box Digitizer/ADC Measuring time differences multiple Mean: times 6499 ( ps > ± ps ) Calculation true time difference between the 2 pulses (mean) FWHM: 19.4 ps Calculation standard deviation of time differences (RMS) In PET: Full Width at Half Maximum (FWHM) of the Gaussian-distribution

4 Digital Leading-Edge (LE) Discriminator Sampling Speed: 1 GS/s Voltage [mv] Voltage [mv] 0 0 0ns 0ns Threshold 1 TH1 Threshold 3 TH3 Threshold 2 TH2 ~22ns 17ns 29.8ns 23.4ns ~31ns Signal 1 Time [ns] Signal 2 Time [ns] Time difference between the 2 Pulses: Peak of Pulse: 9.3 ns ± 0.7ns digital LE TH1-TH2 : 6.4 ns << 0.7ns digital LE TH1-TH3 : 12.8 ns << 0.7ns 74ns Using more points digital of the leading signals, edge: will increase timing resolution Baseline (FWHM) restore Ideal signals Find the will 2 always sampling have points same around amplitudes, TH - fix THs Calculate result always time with in same lin. Interpolation time difference - no Time Difference Walk between TH1 and TH2 Changing results THs in result time difference in different of time the differences signals 74ns

5 Domino Ring Sampler DRS Theory ns Inverter Domino ring chain IN Clock Shift Register Waveform stored Out FADC 33 MHz Dead time during readout Time stretcher GHz MHz

6 In Used Digitizers DRS4 FPGA ADC DT5742 from CAEN CAEN Same like PSI 16 channel (2 DRS4-chips synch.) DRS4-Evaluation Board 4 channel 1024 sampling points/channel PSI 700 MHz BW up to 5 GS/s digital oscilloscope 12 bit resolution LeCroy WaveRunner Zi640 SCOPE 8 bit up to 20 GS/s 4 GHz BW 4 channel 6 Department of Preclinical Imaging and Radiopharmacy

7 ADC vs. DRS4 for PET Use TI-ADS5400, 12bit, 1-GS/s, 2.1 GHz BW: DRS4-chip, 12bit, up to 6 GS/s, 0.9GHz BW: Continuously sampling Large dead time Equidistant sampling points Time calibration information needed High power consumption ~2.2 W Low power consumption ~30 mw

8 Results of Different Digitizers

9 Ideal Signals Measurements Function Generator Delay Box Digitizer/ADC SCOPE 2GS/s PSI 5GS/s CAEN 5GS/s Why is PSI worse than CAEN? Why is SCOPE better? delay cable (ns)

10 Timing Resolution Depends on Calibration ns 1.02 ns TH ns 1.09 ns Channel 1 0 TH1 Channel 2 0

11 Timing Resolution Depends On Time Differences Fit (35 points used) SCOPE 2GS/s PSI 5GS/s CAEN 5GS/s SCOPE 2GS/s PSI 5GS/s CAEN 5GS/s delay cable (ns) delay cable (ns) Better Calibration- Method for DRS4-Chip is Needed! Why does the FWHM depends on the time difference or delay cable length of the signals? 66 MHz pattern for the PSI-board and a 50 MHz pattern for the CAEN-board à Cross Talk from FPGA clock

12 Calibrating the DRS4-Chip at 5GS/s Searching for values of the rising edges, that matches within a given error (here ± 4mV) The time difference between the two capacitors must be a multiple of the sampling frequency Cell 25 is 3 x 4440 ps away from Cell 94 Cell 73 is 3 x 4440 ps away from Cell 142 and 5 x 4440 ps from Cell 189 Changing 40 times the sine wave frequency in 10 ps steps will provide a max. calibration range of 390 ps

13 Comparison Old vs. New Calibration for 5GS/s DRS4-chip Calibration: NEW time difference between 2 cells [ns] DRS4-chip Calibration: CAEN Same DRS4 chip time difference between 2 cells [ns]

14 Comparison Old vs. New Calibration for 5GS/s DRS4-chip Calibration: NEW PSI : 1024 cells 195 ps ± 3 ps CAEN: 1024 cells 200 ps ± 3 ps NEW : 1024 cells 200 ps ± 74 ps 512 cells 270 ps ± 23 ps 512 cells 130 ps ± 22 ps time difference between 2 cells [ns] New Calibrated Time Line DRS4-Chip DRS4-chip Calibration: CAEN Other DRS4-chip Calibration: PSI time difference between 2 cells [ns] time difference between 2 cells [ns]

15 Getting Different Time Differences Reminder: Time difference between the 2 Pulses: Voltage [mv] Threshold 1 TH1 29.8ns Signal 1 digital LE TH1-TH2 : 6.4 ns << 0.7ns digital LE TH1-TH3 : 12.8 ns << 0.7ns ns 0ns Time [ns] 74ns Voltage [mv] 0 0ns Threshold 3 TH3 Threshold 2 TH2 17ns Signal 2 Time [ns] Changing THs result in different time differences 74ns

16 Timing for Ideal Signals: New vs. Old Cal. Old Cal. DRS4: ps FWHM [ps] New Cal. DRS4: 19 ps FWHM [ps] Not satisfying calibrated Timing Resolution (FWHM) depends on Time Difference After Calibration the error (FWHM) is reduced from fluctuating 50-90ps to 19ps Time Diff. [ps]

17 Timing for Ideal Signals: DRS4s vs. ADC PSI: 22 ps FWHM [ps] CAEN: 19 ps FWHM [ps] SCOPE: 25 ps FWHM [ps] 2 DRS4 synch.: 26 ps FWHM [ps]

18 Conclusions With the new DRS4-chip calibration the error was reduced to a minimum The DRS4-chip showed similar results as high speed ADCs in respect to timing resolution, even across multiple DRS4- chips in coincidence As for power consumption, the ADC is inefficient due to its brute force method, whereas the DRS4 chip is an advanced technology for HEP and medical imaging For high resolution TOF-PET systems a validated and calibrated readout electronics is needed: the upcoming DRS5 chip is a promising candidate Higher analog BW Higher sampling rate Dead time free

19 Thank You For Your Attention Acknowledgements Christoph Parl Chih-Chieh Liu Frederic Mantlik Mathwew Devine Armin Kolb Matthias Schmand Nan Zhang Joe Camp 19 Department of Preclinical Imaging and Radiopharmacy

20 Comparing 2 Calibrated DRS4-Chips

21 Coincidence Timing: Test Setup PET Signals 5x5x5 mm Dark-Box 5x5x5 mm PMT1 LSO Na22 LSO PMT2 PC DRS4 Evaluation Board ch1 ch2 PMT Hamamatsu H Department of Preclinical Imaging and Radiopharmacy

22 2 PMTs with LSO:Ca New Calibration 2 PMTs DRS4,New Cal. 5 GS/s DRS4-chip gives similar timing resolution then 20 GS/s SCOPE/ADC New Cal. CAEN: 173 ps FWHM [ps] 2 PMTs SCOPE 20 GS/s SCOPE: 169 ps FWHM [ps]