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Simultaneous Photon Counting and Charge Integrating Readout Electronics for X-ray Imaging Hans Krüger, University of Bonn, Germany SI LAB Silizium Labor Bonn University of Bonn: Michael Karagounis, Manuel Koch, Edgar Kraft, Hans Krüger, Norbert Wermes University of Mannheim: Peter Fischer, Ivan Peric Philips Research Laboratories Aachen: Christoph Herrmann, Augusto Nascetti, Michael Overdick, Walter Rütten FEE2006, Perugia SI LAB Motivation Silizium Labor Bonn Photon counting • limited to count rates < 10 MHz / pixel • Quantum limited noise statistics Charge integration • High photon flux • does not reach quantum limited resolution at low photon flux photon counter integrator sim. counting and integrating (CIX) # photons yes no yes total energy no yes yes low flux yes no yes high flux no yes yes spectral information no no yes FEE2006, Perugia Hans Krüger, University of Bonn (mean photon energy) 1 Counting and Integrating X-ray Detection (CIX) SI LAB Silizium Labor Bonn more information from the same x-ray dosage Integrator Photon counter signal intensity FEE2006, Perugia Hans Krüger, University of Bonn 2 SI LAB Pixel Concept Silizium Labor Bonn Conversion Layer Total deposited energy Integrating Channel Counting Channel Preamp FEE2006, Perugia Hans Krüger, University of Bonn Mean photon energy Number of absorbed photons 3 SI LAB Silizium Labor Bonn Implementation FEE2006, Perugia Hans Krüger, University of Bonn 4 Prototype chip CIX 0.1 SI LAB Silizium Labor Bonn chip features: • AMS 0.35 µm CMOS technology • area per electronics channel: 100 µm 547 µm • linear arrangement of 17 cells (no bump bond pads) 2 test pixels with access to sub-circuits, e.g. preamplifier analog output • in-pixel signal generation circuits (design for testability) • low noise digital logic (low-swing differential current steering logic, DCL) FEE2006, Perugia Hans Krüger, University of Bonn 5 Pixel Cell Block Diagram preamp with continuous reset replication of feedback current sourced to the integrator Charge integration (I to F converter) - Silizium Labor Bonn Photon counting - SI LAB comparator output triggers charge pump (synchronous) constant charge packet removed from integrator feedback capacitor Cint number of pump cycles and timestamps for first and last cycle stored Signal simulation - switched capacitor and switched current charge injection circuits internal/external dc current source FEE2006, Perugia Hans Krüger, University of Bonn 6 SI LAB Integrator: Charge Packet Counting UCINT UCINT Time320µs small current Silizium Labor Bonn fclk = 8 MHz pump cycles = 2 Time = 2560 Imeas [pkts./clk] = 1/2560 = 0,0004 t Time(1/3)*320µs larger current fclk = 8 MHz pump cycles = 2 Time = 853 Imeas [pkts./clk] = 1/853 Frame=320µs FEE2006, Perugia t Hans Krüger, University of Bonn = 0,0012 7 SI LAB Feedback Circuit Silizium Labor Bonn 3 differential pairs: • continuous reset of the CSA feedback capacitor • signal replication to source the integrator • leakage current compensation leakage current compensation feedback (slow) (fast) FEE2006, Perugia Hans Krüger, University of Bonn 8 SI LAB Charge injection circuits (chopper) Silizium Labor Bonn • chopper 1+2: switched capacitors (~10 fF), connected to preamplifier input • current chopper: switched current source (800 nA max.), connected to preamplifier or integrator input minimal pulse duration ~30 ns • leakage current simulation • up to five load capacitors (~100 fF each) connected to preamplifier input Chopper 1 current chopper IntIn VDDChopper /IntCurrInjEn VCal Leakage current simulation to integrator Input capacitance simulation to preamplifier /AmpCurrInjEn /Str1 CascBias Chopper 2 ICurrInjN /Str2 SelLoad0..4 VDDA ILeakSimN /LeakSimEn Cinj /ChInjEn AGND AGND VDDChopper StrCurr FEE2006, Perugia Hans Krüger, University of Bonn 9 SI LAB Integrator and Charge Pumps Silizium Labor Bonn • switched capacitor charge pump: dQ = (VDDA - VIntRef) · 240 fF, typical charge packet 1.8 · 106 e- (i.e. 140 60 keV photons or 170 photons at 120 keV tube spectrum) 1.7µA maximal current throughput (at 6 MHz clock rate) • switched current charge pump packet size controlled by IPump bias DAC and clock rate • VIntTh controls charge pump trigger level /Reset IPumpP CurrPump /CurrPump current pump 240f VDDA ChPumpRes ChPump capacitive pump 300f CompOut VIntRef VIntTh Integrator FEE2006, Perugia Hans Krüger, University of Bonn Comparator 10 SI LAB Differential Current Mode Logic Silizium Labor Bonn Differential pair with constant bias current Ibias Ibias - loads generate low voltage swings by I to U conversion in in out out An ‘ideal’ load characteristic: - Vhi level fixed at maximum possible input voltage (~VDD-Vth –VDsat) - Vlow level fixed by the voltage swing required to ‘fully’ switch the current in the cell (~200 mV) - plateau at ½ Ibias guarantees equal rise and fall times - and all this independent of the absolute value of Ibias to match given loads and speed requirements Load IU CML principle (inverter) Iload Ibias ½ Ibias Vlo P. Fischer, E. Kraft, “Low swing differential logic for mixed signal applications”, Nucl. Instr. Meth. A 518 (2004) 511-514 FEE2006, Perugia Load IU Hans Krüger, University of Bonn Vhi Vload 'ideal' load characteristic 11 SI LAB Implementation of the load circuit Silizium Labor Bonn Approximation of the ideal load circuit - in NMOS operated as a current source with adjustable voltage VSS diode connected NMOS (or pn-diode) to ground Vhi increases only little with Ibias Differential swing can be adjusted through VSS measured load characteristic 2,0 Iload [µA] bias VSS GND Iload VSS=0V, Bias A VSS=0V, Bias B VSS=0.2V, Bias A VSS=0.2V, Bias B 1,5 Load IU ½ Ibias 1,0 Vlo Vhi Vload 0,5 DAC=15/31 0,0 0,0 FEE2006, Perugia 0,1 0,2 Vload [V] 0,3 0,4 0,5 Hans Krüger, University of Bonn 12 SI LAB Silizium Labor Bonn Measurements FEE2006, Perugia Hans Krüger, University of Bonn 13 Photon Counter Performance minimum operational threshold 500 e equivalent noise charge 180 e + 79 e / 100 fF maximum count rate 6 (12) MHz with static (dynamic) leakage current compensation double pulse resolution >100 ns FEE2006, Perugia Hans Krüger, University of Bonn SI LAB Silizium Labor Bonn 14 SI LAB Integrator Noise Performance Silizium Labor Bonn discretisation limit Poisson SNR limit perfect 12-bit ADC FEE2006, Perugia Hans Krüger, University of Bonn 15 Impact of the Feedback Circuit SI LAB Silizium Labor Bonn DIRECT injection via feedback Poisson SNR limit noise performance not optimal but Poisson statistics limits SNR for real X-ray photon detection (60 keV X-rays, CdTe sensor, 320 µs frame time): - 100 pA 23 ph, sqrt(23) = 4,8 - 1 nA 226 ph, sqrt(226) = 15 - 10 nA 2260 ph, sqrt(2260) = 48 FEE2006, Perugia Hans Krüger, University of Bonn 16 SI LAB Total Dynamic Range Silizium Labor Bonn 200 nA integrator 12 nA 6 MHz max. overlap region pulse frequency 66 pA photon counter a single pulse FEE2006, Perugia Hans Krüger, University of Bonn 3 pA 17 Reconstruction of the Mean Photon Energy total energy / photon count SI LAB Silizium Labor Bonn photon counter overload original pulse size integrator lower limit FEE2006, Perugia Hans Krüger, University of Bonn 18 SI LAB Summary Silizium Labor Bonn A readout scheme which is capable of simultaneous counting and integrating absorbed X-ray quanta has been proposed and implemented The multi-stage feedback circuit of the pre-amplifier mirrors the signal current to the integrator and provides leakage current compensation A prototype chip has been submitted and tested and showed the feasibility of the concept The simultaneous operation is fully functional though still impaired by the excess noise of the (not optimized) feedback network A new test chip has been submitted and is currently under study Acknowledgements: Edgar Kraft for the animated ppt – sildes References: • E. Kraft et al., “Counting and Integrating Readout for Direct Conversion X-ray Imaging - Concept, Realization and First Prototype Measurement”, Proceedings of the IEEE 2005 NSS/MIC • P. Fischer, E. Kraft, “Low swing differential logic for mixed signal applications”, Nucl. Instr. Meth. A 518 (2004) 511-514 FEE2006, Perugia Hans Krüger, University of Bonn 19
