FRONT-END ELECTRONICS PART 1 - Istituto Nazionale di...

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FRONT-END ELECTRONICSPART 1

Francis ANGHINOLFIWednesday 28 March 2007

[email protected]

INFN Laboratori Nazionali di Legnaro, 26-30 Marzo 2007

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IntroductionIn this lecture we will give an insight into electronic signal processing, having in mind the application for particle physics.

• Specific issue about signal processing in particle physics

• Description of a typical “front-end” channel for particle physics detector

• Some examples

In the next lecture, there will be an approach of the “noise” problem :

• Time vs. frequency signal and circuit representation

• Noise sources in electronics circuit & Introduction to the formulation of Equivalent Noise Charge (ENC) in case of circuits used for detector signals.

• Some practical circuits examples

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CREDITS :

Dr. Helmut SPIELER, LBL Laboratory

Dr. Veljko RADEKA, BNL Laboratory

Dr. Willy SANSEN, KU Leuven

REFERENCES

Low-Noise Wide-Band Amplifiers in Bipolar and CMOS Technologies, Z.H. Chang, W. Sansen, Kluwer Academics Publishers

Low-Noise Techniques in Detectors, V. Radeka, Annual Review of Nuclear Particle Science 198828: 217-277

Pierre JARRON, CERN

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Front-End Electronics

• Introduction

• Detector Signal collection

• Preamplifiers & Shapers

• Considerations on Detector Signal Processing

• Examples of front-end circuits

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Introduction

“THE SHAPE, MAGNITUDE AND NOISE CONTENT OF THE INPUT SIGNAL TO THE MAIN AMPLIFIER ARE DETERMINED BY THE DETECTOR AND PREAMPLIFIER” Fairstein, Hahn, 1965

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Detector Signal collection

Particle detector collects charges :

A particle crossing the medium generates ionization + ions avalanche (gas detector) or electron-hole pairs (solid-state).

For scintillator detectors, photons are converted to electronics charge by a converter (PMT, solid state converter)

Charges are collected on electrode plates (as a capacitor), building up a voltage or a current

Typical “front-end” elements

The detector IS PART of the electronic circuit

Z+

-

Board, wires, ...

Particle Detector Circuit

Rp A

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Signal characteristics vary with the detector :

-Nanoseconds for solid state detector, no further ionization or avalanche in detector. Conversion ratio is 1 eh pair creation for 3.2eV incident energy in silicium.

-A minimum ionizing particle crossing a silicium detector delivers ~ 25000 electrons (4fC)

-The solid state detector efficiency is 100%

Solid State detector :

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-Microsecond range for (traditional) gas detectors : a particle crossing the gas triggers a ion creation (gas detector) eventually followed by multiple other ionizations (avalanche as in proportional chambers). The primary ionization energy is ~ 25-35 eV per ion. The avalanche does signal amplification in the detector.

-The signal may have a fast component (within 50ns or less) and a slower component (tail extending up to microsecond, ions collection)

-In case of avalanche the secondary ionization may occur with a stochastic distribution in time and position along the primary ion track : the resulting signal is made of a serie of contributions to the total charge. The significant value is the full collected charge : to collect the charge signal integration is necessary.

Gas detector :

Current flux with different ionizations

Integrated current (full charge)

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Gas detector :

-The signal is usually large

Example : the Transition Radiation Detector (TRT)

The detector is a proportional chamber. The charge is collected in a relatively short time scale, with a time function K/(t+t0). The total electronic charge may reach 200fC, distributed in 40-50 ns. However most of the charge is collected at time 0 : a very short shaping time of a few ns is possible and collects ~ 15% of the total charge, with a very good time resolution.

40ns

<10ns

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-Very fast signals are possible with scintillating detectors : a particle generate a photon. The photon is converted to a photoelectron in a photomultiplier tube (PMT) or an avalanche photodiode (APD). These two elements provide further amplification (x106) by further electrons avalanche as in PMT or electron-hole pairs avalanche as for APD).

-The signal obtained is usually huge (> 160fC range)

-The photon/electron conversion is not 100% efficient.

Scintillator detector :

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Electronics functions are multiple :

Signal amplification (signal multiplication factor) (when detector signal is low as for solid state detectors)

Noise rejection

Signal “shaping” : integration as in case of gas detectors

Tail cancellation for detectors with long signal tails


Z+

-


Rp A

Board, wires, ...

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If Z is high, charge is kept on capacitor nodes and a voltage builds up (until capacitor is discharged)

If Z is low charge flows as a current through the impedance in a short time.

In particle physics, low input impedance circuits are used:

• no signal pile up at the input

• limited channel-to-channel crosstalk

• low sensitivity to parasitic signals


Z+

-

Board, wires, ...


Rp

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Detector Signal collectionCircuit

Low Z output voltage source circuit can drive any load

Output signal shape adapted to subsequent stage (ADC)

ZoZ+

-

High Z

Low Z

Low Z

T

Voltage source

• Impedance adaptation• Amplitude resolution• Time resolution• Signal versus Noise

Rp

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Signal shaping is used to :

Reduce noise vs. signal

Limit pile-up (overlap of pulses)

Increase time resolution

Reduce tail

Other parameters are :

Linear range

Dynamic range

Large signal recovery

Power consumption

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A typical detector front-end

Preamplifier Shaper

δδδδ(t) Q/C.ν(t)

I O

What are the functions of preamplifier and shaper (in ideal world) :

• Preamplifier : is an ideal integrator : it detects an input charge burst

Q δ(t). The output is a voltage step Q/C.ν(t). Has large signal gain such that noise of subsequent stage (shaper) is negligeable.

• Shaper : a filter with : characteristics fixed to give a predefined output signal shape, and rejection of noise frequency componentswhich are outside of the signal frequency range.

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Charge Sensitive Preamplifier

Cd

i(t) Charge from detector builds up a voltage on the detector node capacitance

CdQVdet in=

Cf

Amplifier

The preamplifier reacts in such a way that its input node voltage stays unchanged : the output voltage Vout is moving to the point where :

Vout.Cf = -Qin

The input node voltage returns to zero

Cd

Qin

-Qin

(~0)

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Cf

Amplifier

Cd

i in

(~0)

vo

CO

Cf)sRin.(Cd1

1.

sCf

1

i

vo

in ++=

It can be shown that the input impedance of the charge preamplifier is given by :

Frequency domain

gm.Cf

Cf)(CoRin

+=

Input node RC time constant1 2 3 4 5

0.2

0.4

0.6

0.8

1vo

t

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5 10 15 20

0.2

0.4

0.6

0.8

1

5 10 15 20

0.2

0.4

0.6

0.8

1

5 10 15 20

0.2

0.4

0.6

0.8

1


Cf

Amplifier

Cd

i in

(~0)

vo

CO

With Resistive feedback element

Rf

Feedback RC Input node RC

RfCf=500RinCin

RfCf=50RinCin

RfCf=5RinCin Ballistic deficitCf)sRin.(Cd1

1.

s.Rf.Cf1

Rf.

sCf

1

i

vo

in +++=

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5 10 15 20

0.2

0.4

0.6

0.8

1


Cf

Amplifier

Cd

i in

(~0)

vo

CO

Charge collection time

Rf

Feedback RC Input node RC

RfCf=50RinCin

10 20 30 40

0.2

0.4

0.6

0.8

1

Ballistic deficit

Input charge collection time

Instant charge

Cf)sRin.(Cd1

1.

s.Rf.Cf1

Rf.

sCf

1

i

vo

in +++=

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0 . 0 1 0 . 0 5 0 . 1 0 . 5 1 5 1 00 . 0 1 5

0 . 0 2

0 . 0 3

0 . 0 5

0 . 0 7

0 . 1

0 . 1 5

0 . 2

Simple Shaper function

CR-RC s-transfer function

h(s) = RCs/(1+RCs)2

Vout

VinRCjω(

RCjωVout

2)1+=

CR-RC time functionRCteRCtt /)/1()(H −−=

Example RC=0.5 s=jω

1 2 3 4 5

- 0 . 2

0 . 2

0 . 4

0 . 6

0 . 8

1

Vin

R

CR

C

1

Combining one low-pass (RC) and one high-pass (CR) filter :

2 4 6 8 1 0 1 2 1 4

0 . 0 2 5

0 . 0 5

0 . 0 7 5

0 . 1

0 . 1 2 5

0 . 1 5

0 . 1 7 5

Step response

Log-Log scale f

|h(s)|

HighZ Low Z

Impulse response

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2 4 6 8 1 0 1 2 1 4

0 . 0 2 5

0 . 0 5

0 . 0 7 5

0 . 1

0 . 1 2 5

0 . 1 5

0 . 1 7 5

Preamplifier & CR-RC2Shaper

Preamplifier Shaper

CR_RC2 shaperIdeal Integrator

δδδδ(t)

1/s RCs /(1+RCs)2x

I O

T.F. from I to O

= = RC/(1+RCs)2

Output signal of preamplifier + shaper

t

1 2 3 4 5

- 0 . 2

0 . 2

0 . 4

0 . 6

0 . 8

1

1 2 3 4 5

0 . 2

0 . 4

0 . 6

0 . 8

1

0 . 0 1 0 . 0 5 0 . 1 0 . 5 1 5 1 00 . 0 1 5

0 . 0 2

0 . 0 3

0 . 0 5

0 . 0 7

0 . 1

0 . 1 5

0 . 2

0 . 2 0 . 5 1 2 5 1 00 . 1

0 . 2

0 . 5

1

2

5

t

f

t

fQ/C.ν(t)

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Semi-Gaussian Shaper function

CR-RC4 s-transfer function

h(s) = RCs/(1+RCs)5

Vout

VinRCj

RCjVout

n)1( ωω

+=

CR-RC4 time functionRCtetRCtt /3)./4()(H −−=

R

C

Example RC=0.5, n=5 s=jω

Vin

R

C

1

Combining n low-pass (RC) and one high-pass (CR) filter :

0 . 0 0 1 0 . 0 0 5 0 . 0 1 0 . 0 5 0 . 1 0 . 5 1

0 . 0 0 0 1

0 . 0 0 0 2

0 . 0 0 0 5

0 . 0 0 1

0 . 0 0 2

0 . 0 0 5

0 . 0 1

0 . 0 22 4 6 8 1 0

- 0 . 0 0 5

- 0 . 0 0 2 5

0 . 0 0 2 5

0 . 0 0 5

0 . 0 0 7 5

0 . 0 1

2 4 6 8 1 0

0 . 0 0 2

0 . 0 0 4

0 . 0 0 6

0 . 0 0 8

0 . 0 1

0 . 0 1 2

Log-Log scalef

|h(s)|

Step response

R

C

1

n times

Impulse response

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5 1 0 1 5 2 0 2 5 3 0 3 5

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

0 . 1

Preamplifier & Semi-Gaussian Shaper

2 4 6 8 1 0

- 0 . 0 0 5

- 0 . 0 0 2 5

0 . 0 0 2 5

0 . 0 0 5

0 . 0 0 7 5

0 . 0 1

Preamplifier Shaper

δδδδ(t)

1/s RCs /(1+RCs)5x

I O

T.F. from I to O

= = RC/(1+RCs)5

Output signal of preamplifier + shaper

t

1 2 3 4 5

0 . 2

0 . 4

0 . 6

0 . 8

1

t

0 . 2 0 . 5 1 2 5 1 00 . 1

0 . 2

0 . 5

1

2

5

f

t

0 . 0 0 1 0 . 0 0 50 . 0 1 0 . 0 5 0 . 1 0 . 5 1

0 . 0 0 0 1

0 . 0 0 0 2

0 . 0 0 0 5

0 . 0 0 1

0 . 0 0 2

0 . 0 0 5

0 . 0 1

0 . 0 2

f

CR_RC4 shaperIdeal Integrator

Q/C.ν(t)

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Preamplifier & Shaper

Vout

Cf

Schema of a Preamplifier-Shaper circuit

N IntegratorsDiff

Semi-Gaussian Shaper

Cd T0 T0 T0

Vout(s) = Q/sCf . [sT0/1+ sT0].[A/1+ sT0]n

Vout(t) = [QAn nn /Cf n!].[t/Ts]n.e-nt/Ts

Peaking time Ts = nT0 !

Output voltage at peak is given by :

Vout shape vs. n order,renormalized to 1

Vout peak vs. n2 3 4 5 6 7

0 . 2

0 . 4

0 . 6

0 . 8

1

Voutp = QAn nn /Cf n!en

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Pile-up :

A fast return to zero time is required to :

• Avoid cumulated baseline shifts (average detector pulse rate should be known)• Optimize noise as long tails contribute to larger noise level

2 4 6 8 1 0 1 2 1 4

0 . 0 2 5

0 . 0 5

0 . 0 7 5

0 . 1

0 . 1 2 5

0 . 1 5

0 . 1 7 5

2nd hit

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Pile-up

• The detector pulse is transformed by the front-end circuit to obtain a signal with a finite return to zero time

2 4 6 8 1 0 1 2 1 4

0 . 0 2 5

0 . 0 5

0 . 0 7 5

0 . 1

0 . 1 2 5

0 . 1 5

0 . 1 7 5

5 1 0 1 5 2 0 2 5 3 0 3 5

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

0 . 1

CR-RC2 :Return to baseline > 7*Tp

Quasi-Gaussian :Return to baseline < 3*Tp

Tp

Tp

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Preamplifier Pile up

Preamplifier Shaper

δδδδ(t) Q/C.ν(t)

I O

What are the functions of preamplifier and shaper (in ideal world) :

• Preamplifier : if it is an ideal integrator : the charges accumulate and the output goes to saturation

• Shaper : pileup occurs as well and distorsion (non linearity) appears when the input approaches saturation limits

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5 1 0 1 5 2 0

0 . 0 1

0 . 0 2

0 . 0 3

Preamplifier Pile up resolution

Preamplifier Shaper

CR_RC2 shaperNon-Ideal Integratorδδδδ(t)

1/(1+T1s) RCs /(1+RCs)2

I O

T.F. from I to O

x

Non ideal shape, long tail

Integrator with a slow decay time

Long tails contributes to additional noise & pileup effects at the shaper output

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2 4 6 8 1 0 1 2 1 4

0 . 0 2 5

0 . 0 5

0 . 0 7 5

0 . 1

0 . 1 2 5

0 . 1 5

0 . 1 7 5


Preamplifier Shaper

δδδδ(t)

1/(1+T1s) (1+T1s) /(1+RCs)2

Pole-Zero Cancellation

I O

T.F. from I to O x

CR_RC2 shaperNon-Ideal Integrator

Ideal shape, no tail

Integrator with slow decay time

Pole-Zero Cancellation

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Vout

Schema of a Preamplifier-Shaper circuitwith pole-zero cancellation

Vout(s) = Q/(1+sTf)Cf . [(1+sTp)/1+ sT0].[A/1+ sT0]n

By adjusting Tp=Rp.Cp and Tf=Rf.Cf such that Tp = Tf, we obtain the same shape as with a perfect integrator at the input

Rf

CfN IntegratorsDiff

Semi-Gaussian Shaper

CdCp

T0 T0

Rp

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Transimpedance Preamplifier

Cd

Rf

Detector model

Amplifier

Resistive feedback element

Vout

i(t)

)i(t)dt(Qt

0

in ∫=

)(i.RVout(t) inf t=

The instantaneous output voltage is the image of the current flow at the detector output (ideal case !)

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RO


Rf

Amplifier

Cd

i in

(~0)

vo

CO

o)sRo.1sRin.Cin)((1

1.Rf

i

vo

in C++≈

The input impedance of the transimpedancepreamplifier can be very low (a few ohms) :

0A

RfRin =

• The transfer function allows fast shaping time

• However stability is a major issue

• The input impedance can be very low (10-100 ohms)

A0

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Rf

Amplifier

Cd

i in

(~0)

vo

CO t2´ 10-7 4´ 10-7 6´ 10-7 8´ 10-7 1´ 10-6

0.00002

0.00004

0.00006

0.00008

0.0001 vo

• The transimpedance front-end already does some “shaping” function

• Additional shaping stages are usually necessary to have quasi-Gaussian shapes and signal gain

Rf=30K, Ro = 500K, Co=0.1pF, Cd=200pF gm=1mS

RO

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1´ 10-7

2´ 10-7

3´ 10-7

4´ 10-7

5´ 10-7

-0.0005

-0.00025

0.00025

0.0005

0.00075

0.001


Rf

Amplifier

Cd

i in

(~0)

vo

CO

vo

t1fC response

Rf=30K, Ro = 500K, Co=0.1pF, Cin=20pF gm = 3mS

The circuit is close to instability

Amplifier

Cd

i in

(~0)

vo

CO

t

Full transfer function

RfCf

1´ 10-8 2´ 10-8 3´ 10-8 4´ 10-8 5´ 10-8

0.0005

0.001

0.0015

0.002

1fC response

vo

Same circuit with Cf=0.1pF compensation capacitance

The circuit is stable

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Preamplifiers comparison

Time response

dependance

Q/Cf

Ballistic deficit

Gain

DifficultEasyStability

100ohms rangeKohms rangeInput Impedance

>5ns>50nsTime constant

TransimpedanceCharge Preamp

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Considerations on Detector Signal Processing

Summary (1)

• The detector pulse is transformed by the front-end circuit to obtain :

• A linear Gain (Vout/Qdet = Cte)

• An impedance adaptation (Low input impedance, low output impedance)

• A signal shape with some level of integration

• A reduction in the amount of electronic noise

• A dynamic range (or Signal-to-Noise ratio)

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Considerations on Detector Signal Processing

Summary (2)

• Very large dynamic range is attainable (16 bits, as for calorimeters)

• Very low noise is achievable in some cases (a few electrons !)

• Peaking time are varying from a few ns (tracking application) to ms range (very low noise systems, amplitude resolution)

• The choice of the suitable front-end circuit is usually a trade-off between key parameters (peaking time, noise, power)

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Examples of front-end circuits

PIXELS

• Highly segmented solid-state detector

• Pixel size as low as 50micronsx50 microns

• Very low occupancy (usually < 10-6)

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PIXELS

• Pixel : 100ns shaping time, 180 el ENC, <1pF detector

Typical Front-End Schematic :

• Charge sensitive preamplifier• Pulse shape variable with input charge• Very high Signal-to-Noise ratio• Very low power (40µW)

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PIXELS FOR IMAGING/MEDICAL APPLICATIONS

256 x 256 pixel

sensitive area ~2 cm2

3-side buttable / Daisy-chain

serial readout ~ 9.2 ms @100MHz Clock

~300 µs 32bit CMOS Parallel Port

14111µµµµm

1612

0 µµ µµm

MEDIPIX2 Chip

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PIXELS FOR IMAGING/MEDICAL APPLICATIONSMEDIPIX2 Chip

• charge sensitive preamplifier with individual leakage current compensation• 2 discriminators with globally adjustable threshold• 3-bit local fine tuning of the threshold per discriminator• 13-bit pseudo-random counter

Preamp

Disc1

Disc2

Double Disc logic

Vth Low

Vth High

13 bits

Shift Register

Input

Ctest

Testbit

Test Input

Maskbit

Maskbit

3 bits

threshold

3 bitsthreshold

Shutter

Mux

Mux

ClockOut

Previous Pixel

Next Pixel

Conf8 bits configuration

Polarity

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SILICON STRIPS DETECTORS

• Strip : 25ns shaping time, 1000 el ENC, <20 pF detector

• Moderate occupancy (10-4)

• Transimpedance type of preamplifier• 75 ns time resolution• Approx. 2mW per channel

ATLAS Silicon Strip detector element : one side has 6 front-end chips (768 independent channels)

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STRAW TUBES (Tracking Gas Detetectors)

• Signal formation : 40ns fast charge, large charge above 200fC (noise not a problem), slow ion drift time contributes to a longsignal tail

• Large occupancy (1 m long object)

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STRAW TUBES (Tracking Gas Detetectors)

10 ns

• The front-end electronics has a short shaping time (7ns), to avoid pile-up. Only 15% of the charge is captured

• Long ion tail signal is cancelled by a baseline restorer circuit

A B

A

B

ASDBLR circuit (M. Newcomer, Penn U.)

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• Amplify, shape, store and digitize signals

– 16 bits dynamic range current preamps

– Trigain (1-10-100) CRRC2 shaper

– Fast shaping time to avoid pile-up

CALORIMETER

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