Readout Electronics for Pixel Sensors

Readout Electronics for Pixel Sensors

• Integrating ASICs for Scintillator-coated X-ray pixel sensors.

• Integrating ASICs for Ion beam profiling.

• Pixel electronics for photon counting .

ASICs for scintillator-coated X-ray pixel sensor

• Pixel sensor based on p-diffusion/n-well photodiode with in-pixel preamplifier (NIM-A)

– Improved optical sensitivity.

– Low direct X-ray absorption.

• Phototransistor-based pixel sensor, (IEEE ICECS’2k1)

– Cancellation of the dark current was achieved.

– Work should be done to improve the FPN.

• Monte Carlo simulation of imaging properties,

(NSS’2000).

ASICs for Ion Beam Profiler

• A pixel array readout electronics was fabricated and tested.

– 10x10 array size.

– 520x520 um pixel size.

– Chip size 6.5x6.5 mm² . – Current integrating mode.

• A new version with improvements includes:

– Variable dynamic range by multiple divisions of the input current.

– Improved output current readout with a transistor switch.

– A 400 pixel array will be assembled.

Microphotograph of part of the chip showing some pixels and bonding pads Chip photograph of the ion beam ASIC

ASICs for Ion Beam Profiler

A photograph of the graphite detector mounted to the flange

Sequence of images showing the ion beam moving across the graphite detector. The data was taken by the ASIC chip.

Pixel detector for Ion Beam Profiler

• A new biasing method for preamplifier-shapers, (IEEE ICECS’2k).

• An all-analogue time-walk free SCA for photon counting pixel sensors, (WSES/IEEE CSCC’2k1)

• An All-digital window discriminator, (IEE Letters)

• Low digital interference counter for photon counting pixels.

Pixel Electronics for

Photon Counting

• CMOS technology is attractive in ASICs.

• CSA is widely used in front-end electronics for radiation detectors.

• For a good noise performance R

must be very high (Mega/Giga Ohm).

• MOS transistor M

is operated in the linear or subthreshold range to reach high Rds values.

• A special bias for M

is required.

A new biasing method for preamplifier-shapers

- A C_f R_f

I_in

V_out M_f

Circuit Description

General analog pulse processing channel

- A -A1 -A2 -An

Rf Cf

Pole-zero cancellation differentiation Charge-sensitive preamplifier

Amplification Nth order integration

detector

Cd

Circuit Description

D 1 I p V D D

C f

M F C f ’

M F ’ + 1 + 1

M s

M 2 I s

M 1

M a i n C S A M i r r o r C S A

Circuit Description

A block diagram of the CSA-shaper showing the mirror bias circuit s h a p i n g t i m e c o n t r o l

g a i n c o n t r o l

f b r e s i s t o r c o n t r o l s h a p i n g t i m e c o n t r o l

M a

M i r r o r C S A - s h a p e r

- A - A

M a i n C S A - s h a p e r

M b C 1

Circuit Description

Simulation Results

The CSA output response for a charge pulse (0.1 fC) at various temperatures (- 65 ^oC to 25 ^oC).

The temperature range represent threshold

variations.

CSA output at - 65 ^oC

CSA output at 25 ^oC

Simulation Results

The output pulse shape is fixed regardless

Shaper output for various Rds values

Higher and lower Rds

(CSA output)

An all-analogue SCA for photon counting pixels

Schematic diagram of the all-analog SCA

C_i M o n o s t a b l e

M S 2

M o n o s t a b l e M S 1 l o w e r th r e s h o l d

u p p e r t h r e s h o l d

i n p u t U L D

L L D

V D D

V S S I_{re f}

R e s e t M r

M 1

M n 1 M p 1

M 2 T

T

I_{re f} V b

4 v o lt s

An all-analogue SCA for

photon counting pixels

Conclusions

• The scintillator-coated pixel sensor using a p-diff/n-well photosensor with an in-pixel preamplifier has the best performance (good sensitivity and low noise).

• The ASIC chip for ion beam profiling will be modified to allow a flexible dynamic range by employing a current gain control.

A better output current readout control will be implemented.

• Circuit mirroring is an efficient method for achieving a stable

operation of MOS transistors in the non-saturation mode by

providing respectively adaptive nodes that vary in the same

manner as the main circuit.

Conclusions

• An all-analog pixel design would result in a reduced pixel size and low electronic noise (e.g. a 14-bit counter will occupy 14000um² in a 0.8 CMOS process while a 2pF capacitor will need 500um² only.

The design will also result in a higher readout speed since the serial data shifting of the traditional in-pixel counter is eliminated.

• The implementation of an asynchronous design of the window discriminator logic will achieve an overall area-efficient photon counting pixel..

• The design of a prescaled shift-counter in a photon counting pixel significantly reduces the noise caused by the switching activity of the digital part in the pixel circuit.

The END

The END