1advanced analog circuit design analog electronics – general introduction analog – continuous...

1Advanced analog circuit design

Analog electronics – general introduction Analog – continuous in time Digital – discrete in time Design of amplifiers and filters ADCs Logic gates Receivers, transmitters Storage cells

Filter ADC

001010100

Amplifier DSP

S E

L

01011

Sensor


Analog electronics – general introduction

Digital design: compromise between power consumption and processing speed Analog design: compromise between speed, power consumption, resolution,

supply voltage, linearity… Analog circuit are crosstalk and noise sensitive Analog design can‘t be automatized Different levels of abstraction

B

A

G

D S

Transistor

PMOS

Verstärker System


Analog electronics in scientific applications Particle detectors with high spatial resolution

- Semiconductor detectors with spatial resolution are today widely used in consumer digital cameras, professional HDTV cameras, medical imaging and in science-grade instruments for particle physics, astronomy, material and biology studies (x-ray diffraction imaging, electron-microscopy) and many other fields.

- Spatial resolution of semiconductor detectors is achieved by segmenting the sensor surface into many small picture elements ("pixels"). Every segment has its own signal collecting region that can be readout individually.

- These detectors are distinguishable from the sensors for consumer electronics either by its low noise and single-particle detection capability or by other properties such as 100% fill-factor, high time resolution, high dynamic range, radiation tolerance, etc.

Multi-channel systems Pixel electronics Signal amplification, signal transmission, sampling, comparison, A/D conversion,

time-measurements, amplitude measurement Amplifiers, filters, switched-voltage/current circuits, comparators, A/D convertors,

oscillators… AC analysis, feedback Transistor models Noise, threshold dispersion Semiconductors – solid state physics


Amplifier

Comparator

Hit memory

Filter

SRAM

DAC

P-”guard-ring”

N-well

55 μm

Pixel electronics


Pixel sensors for particle physics

Pixel sensors are used to detect high-energy charged particles, and to determine particle trajectories.

Since particles tracking requires many layers of planar detectors, tracking sensors should be as transparent for particles as possible. They should be very thin, otherwise the particles will be deflected from their initial trajectories.

Silicon is the best material for such detectors since silicon-based technologies offer the possibility to implement any possible semiconductor device (from PN junction to the completed signal processing electronics) on the sensor.


Pixel-sensors for medical imaging

In the case of high energy photon (x-ray or gamma) detection for medical imaging, the requirements are opposite. Photon sensors should be thick enough to absorb the largest part of the radiation. Due to its low absorption coefficient, silicon is not the best material for high-energy photon detection.

The most of practical pixel sensors for such radiation are based on indirect detection. Such sensors consist of a layer of scintillator material that converts the high-energy photons into visible light. The light detection is then performed by a silicon pixel sensor layer.


Pixel-sensors for medical imaging

SIPMs

Readout chip

FPGA

USB ChipSupply voltages

USB Cable

PCB1

PCB2

PCB3

PCB4

SIPM signals

Digital output signals – time & energy Control

Bias voltages

Digital output signals

Scintillators

g


Classification of pixel-sensors

Hybrid- and monolithic detectors- Monolithic pixel detectors: An n x m pixel matrix is placed on one chip and

usually connected by means of signal multiplexing to n (or less) readout channels placed on the same or different chip. Pixels of a monolithic detector must be equipped with a certain readout electronics that at least perform the simplest tasks such as signal clearing, multiplexing and in most cases the amplification. (Some of monolithic detectors employ even more complex in-pixel signal- processing and data reduction. In this case we are talking about "intelligent" pixels that can e.g. detect particle hits, perform A/D conversion, transmit pixel addresses, perform time measurements, etc.) There are n or less connections between the pixel matrix and the block of readout channels.

- Hybrid pixel detectors: Each pixel on the sensor chip has its own channel on the readout chip. There are n x m connection between two chips.

Technology – custom or specific- The development of such detectors is relatively low-cost since they use

modern commercially available and well characterized CMOS technologies.- Pixel detectors in the technologies that are specially developed or adjusted

for particle (or visible light) detection, like the technologies on high resistance substrate, thick epi-layer, etc.


Hybrid detectors with fully-depleted sensors

P-type Si - depleted

P-type Si - undepleted

n-type collecting region(n-diffusion)

Pixel i

Potential enegry (e-)

Pixel i

P-type Si - depleted

P-type Si - undepleted

Signal collection

Su

bst

rate


Hybrid detectors with fully-depleted sensors

Standard (bump-bonded) hybrid pixel detectors

- The bump-bonded hybrid pixel detectors are used in high-energy physics for particle tracking, and in medicine and synchrotron experiments as direct detectors for x-rays. They are based on a relatively simple pixel sensor (ohmic or with pn junctions) without any pixel electronics and bump-connections between the pixel sensor and the readout pixel chip

- The connection between the sensor and the readout chip is mechanically complex and expensive, especially in the case of small pixel sizes.

Fu

lly-de

ple

ted

sen

sor

Re

ad

ou

t chip

BumpsMin. pitch ~50 μm

Pixel

Signal charge


Pixel matrix

Bonding matrix for one RO-chip

Power/signal supply for RO-chip

RO-chip (in a “gel”-pack)

Hybrid-detector for cell imaging


Capacitive coupled hybrid detector

Sm

art d

iod

e- o

r fully-d

ep

lete

d se

nso

rR

ea

do

ut ch

ip

Pixel

Glue

Signal charge


1.5 mm

Readout chip (CAPPIX)

Sensor chip (CAPSENSE)

Power supplyand cont. signalsfor the sensor

Power supplyand cont. signalsfor the readout chip

Capacitive coupled hybrid detector


3D hybrid-detector

3D-integration is a technology that allows for both vertical and horizontal connection between electronic components placed on different chips (thinned dies) stacked vertically.

Fu

lly-de

ple

ted

sen

sor

Re

ad

ou

t chip

1

Pixel

Signal charge

Re

ad

ou

t chip

2

TSV

Wafer bond

Wafer bond


Standard monolithic detector - MAPS

In the case of a standard monolithic CMOS sensor ("Monolithic Active Pixel Sensor“) - the sensitive area is undepleted epitaxially-grown silicon layer and the charge is spread and separated by diffusion. Some part of the charge is finally attracted by the next well/diffusion.

MAPS

NMOS transistor in p-well N-well (collecting region)Pixel i

Charge collection (diffusion)

P-type epi-layer

P-type substrate Energy (e-)



Pixel rows are consecutively "selected" by connecting their outputs (usually single-transistor amplifier outputs) to column lines. The pixel signals are in this way transported to the readout channels. Such a multiplexing requires at least one electronic switch per pixel implemented with a transistor.

P-type epi-layer

P-type substrate

Signal out

Select(i) Select(i+1)



MAPS are slower and not as radiation tolerant as the hybrid detectors. standard MAPS do not allow implementation of complete set of CMOS electronics

inside pixels (only n-channel FETs - NMOS transistors - can be used)

NMOS transistor in p-well

N-well (collecting region)

Pixel i

P-type epi-layer

P-type substrate Energy (e-)

MAPS with a PMOS transistor in pixel

PMOS transistor in n-well

Signal collectionSignal loss


Enhanced MAPS

INMAPS

NMOS shielded by a deep p-well

PMOS in a shallow p-well

N-well (collecting region)

Pixel

P-doped epi layer


T-well detector and smart diode array

P-substrate

Depleted E-field region

“Smart” diode T-well MAPS

Deep n-well 2. n-well

P-well

NMOS PMOS

Pixel

Deep n-well

Pixel

Epi-layer

“Smart diode” array

Diffusion

Drift

Potential energy (e-)

Potential energy (e-)


SOI monolithic detector

An SOI detector is based on a modified SOI process. SOI detectors use the electronics layer for the readout circuits and the high-resistivity support layer as a fully-depleted (drift-based) sensor. The sensor is typically 300um thick and has the conventional form of a matrix of pn junctions. A connection through the buried oxide is made to connect the readout electronics with the sensor.

Su

pp

ort la

yer

Ele

ctron

ics laye

rB

urie

d o

xide

Connection

Energy (e-)

CMOS pixel electronics


DEPFET monolithic detector

Pixel

PMOS Ext. gate

Int. gate

Clear

Signal collection

Signal clearing

N-substrate (depleted)

P-type backside contact

Potential en. (e-)

Elect. Interact.

Int. gate


SDD monolithic detector

N-doped collecting region

Depleted n-type substrate

Undepleted p-type backside contact

Drift “rings”

Energy (e-)


2.7

mm

ADC channel

Pixel matrix

Monolithic detector - SDA


Amplification

In its simplest form, pixel signal amplification is performed using a single-transistor amplifier. In the case of Field Effect Transistors (FETs), a single-transistor amplifier is sensitive to the voltage change on its input (gate). The charge signal generated by ionization is first collected by the collecting region. The amplifier is coupled with the collecting region by means of DC-coupling (wire) or by use of AC-coupling (capacitance). The conversion factor between the charge signal and the voltage change is the capacitance of the collecting region, referred to as detector capacitance. Clearly the voltage signal will be higher if the collection region has smaller capacitance.

More efficient amplification is achieved by multi-transistor amplifiers. Such amplifiers are typical for hybrid detectors and advanced CMOS monolithic detectors. They are often equipped with feedback circuit which makes the amplification more linear. An example of an amplifier with feedback is the charge sensitive amplifier - CSA. CSA is sensitive only to the charge injected into its input, the capacitance of the input node does not influence the output signal amplitude.


DetectorDetector (equivalent circuit)

Simple voltage amplifier(source follower)

Bias RBias R

Bias VBias V

Out

Out

Charge sensitive amplifier

CdetCdet

Isig Isig

Amplification


Noise An amplifier not only performs the amplification of the input signal; unfortunately

it also introduces electronic noise. Let us explain this: Every amplifier needs to be biased in order to achieve the desired amplification, which means that the amplifier transistor(s) must conduct a certain bias- (DC) current. The signal on transistor's gate will then modulate the current. Thermal motion of the charge carriers inside the transistor active region (channel), leads to bias current fluctuations. These fluctuations are small compared to the bias current itself, but since the bias current is almost always much larger than the signal, its noise can in many cases exceed the signal. A way to decrease the noise is to extend the measurement time (or add a low-pass filter/shaper). Noise signals are random signals with expected value zero and if the measurement takes long time, the average of the noise during measurement interval will in fact approach zero. Most signals, however, have nonzero DC value and they are unaffected by the measurement time.

We could conclude that the detector capacitance does not play any role if we use CSA. This is, however, not true. The noise of a charge sensitive amplifier depends linearly on the detector capacitance. The reason for this is that the negative feedback which cancels the output noise becomes less efficient if the input amplifier node is loaded with a large capacitance.


0.0 500.0n 1.0µ 1.5µ 2.0µ 2.5µ 3.0µ-10.0m

-5.0m

0.0

5.0m

10.0m

15.0m

20.0m

25.0m

Sig

nal [

V]

Time [s]

Noiseless signal Signal with noise

Noise


„Time walk“

0.0 500.0n 1.0µ 1.5µ 2.0µ-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Sig

na

l[V

]

Time [s]

Response to 600 eResponse to 6000 e

Time walk ~ 70 ns

Threshold


KTC Noise

Almost every electronic circuit that employs transistors will be affected by their noise. This holds also for the transistor-based pulsed-reset circuit. During the pulsed reset, i.e. when the reset switch is closed, the potential of the collecting region will fluctuate around the desired reset value due to the thermal noise in the reset transistor. When the reset transistor is turned off, the instantaneous value of the reset voltage will be frozen. The instantaneous value is the sum of the desired reset-voltage and the reset error. The reset error superposes to the signal and leads to a measurement uncertainty. It is interesting to note that the reset noise only depends on the detector capacitance (not on the reset transistor resistance):

σ2v = kT/Cdet, with σ2v variance of the voltage reset error, k Boltzmann's constant, T

temperature and Cdet detector capacitance.


960.0n 980.0n 1.0µ 1.0µ 1.0µ

1.794

1.796

1.798

1.800

1.802

1.804

1.806

Re

setv

olta

ge

[V]

Time [S]

Reset voltage

Reset switch closed Reset switch opened

Desired reset voltage = 1.8 V

Reset error

Reset switch

Reset

Reset voltage1.8 V

Detector c. = 1 fF

KTC Noise


Properties of pixel sensors Properties Pixel size Detector capacitance Noise

- readout amplifier- reset- and bias-resistor noise- The leakage-current noise- σ2v = kT/(gm t).- The magnitude of the noise determines the smallest detectable signal.

Signal to noise ratio (SNR)- SNR is the ratio between a chosen reference signal and the noise. - SNR ~ (gm t)0.5/Cdet

Dynamic range- Dynamic range is the ratio between the greatest undistorted signal (the greatest signal

for which the readout does not saturate) and the smallest detectable signal (determined by the noise).

Time resolution Power consumption

- FOM = P t / SNR2

Radiation tolerance Fixed pattern noise

- FPN refers to a non-temporal spatial noise and is due to device mismatch in the pixels and/or readout channels.

Radiation length

1advanced analog circuit design analog electronics – general introduction analog – continuous...

Documents

linearity analog circuit

noise sensitive analog

processing speed analog

particle physics pixel

time design of amplifiers

photon sensors

consumer electronics

particle trajectories