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Motivation

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Simultaneous Photon Counting and Charge Integrating Readout Electronics for X-ray Imaging Hans Krüger, University of Bonn, Germany

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

- 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

Hans Krüger, University of Bonn

more information from the same x-ray dosage

Integrator

Photon counter

signal intensity

Hans Krüger, University of Bonn

Conversion Layer

Total deposited energy

Mean photon

energy

Integrating Channel

Counting Channel

Number of

absorbed photons

Preamp

Hans Krüger, University of Bonn

- Implementation

Hans Krüger, University of 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)

Hans Krüger, University of Bonn

- Photon counting
- preamp with continuous reset
- replication of feedback current sourced to the integrator

- Charge integration (I to F converter)
- 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

Hans Krüger, University of Bonn

UCINT

Time320µs

small current

t

UCINT

Time(1/3)*320µs

larger current

t

Frame=320µs

fclk = 8 MHz

pump cycles = 2

Time = 2560

Imeas[pkts./clk] = 1/2560

= 0,0004

fclk = 8 MHz

pump cycles = 2

Time = 853

Imeas[pkts./clk] = 1/853

= 0,0012

Hans Krüger, University of Bonn

- 3 differential pairs:
- continuous reset of the CSA feedback capacitor
- signal replication to source the integrator
- leakage current compensation

leakage current compensation

(slow)

feedback

(fast)

Hans Krüger, University of 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

Hans Krüger, University of 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

Hans Krüger, University of Bonn

Ibias

in

in

out

out

Iload

Load

I U

Load

I U

Ibias

½ Ibias

Vlo

Vhi

Vload

'ideal' load characteristic

- Differential pair with constant bias current Ibias
- loads generate low voltage swings by I to U conversion

- 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 Ibiasto match given loads and speed requirements

CML principle (inverter)

P. Fischer, E. Kraft, “Low swing differential logic for mixed signal applications”, Nucl. Instr. Meth. A 518 (2004) 511-514

Hans Krüger, University of Bonn

in

bias

VSS

GND

- Approximation of the ideal load circuit
- 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

Load

I U

measured load characteristic

Iload

½ Ibias

Vlo

Vhi

Vload

Hans Krüger, University of Bonn

- Measurements

Hans Krüger, University of Bonn

Hans Krüger, University of Bonn

discretisation limit

perfect

12-bit ADC

Poisson SNR limit

Hans Krüger, University of Bonn

- 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

DIRECT injection

via feedback

Poisson SNR limit

Hans Krüger, University of Bonn

200 nA

integrator

12 nA

6 MHz max.

pulse frequency

overlap region

66 pA

photon counter

a single pulse

3 pA

Hans Krüger, University of Bonn

photon counter

overload

original pulse size

integrator lower limit

total energy / photon count

Hans Krüger, University of 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

Hans Krüger, University of Bonn