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Transcript
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
Time320µs
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
IU
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
IU
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
IU
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