Sensor Systems Quiz

Sensor Systems Basics

• A sensor system consists of a sensing part and a processing part, which interacts with a processor (Micro-Controller Unit, MCU) through a single access interface.
• Sensors acquire signals that describe physical phenomena and send the information to the micro-controller.
• A Smart Sensor is a sensor that embeds an Analog-to-Digital converter and a small processing unit that can communicate through simple communication protocols.

Sensor Definitions

• Transducer: a device that transforms energy from one type to another, even if both energy types are in the same domain.
• Sensor: a device that monitors a parameter of a system, without disturbing that parameter.
• Actuator: a component of machines that is responsible for moving or controlling a mechanism or system.

Sensor Characterization

• Sensitivity: the ratio between the change in the output signal to a small change in the input physical signal.
• Resolution (LSB): the smallest increment of measure that a device can make.
• Full Scale Range (FSR): the maximum measurable interval, after which the measure saturates.
• Number of bits (n): in sensors with a digital output, 2n is the number of levels in which the FSR is divided.
• Accuracy: the error between the result of a measurement and the true value.
• Precision (or Repeatability): the sensor's ability to output the same value for the same input over several trials.
• Linearity: the deviation of the output from a best-fit straight line for a given range of the sensor.
• Transfer Function (frequency response): the relationship between the physical input signal and the electrical output signal.
• Bandwidth: the frequency range between the lower and upper cutoff frequencies, within which the sensor transfer function is constant gain or linear.
• Noise: the random fluctuation in the measured value.
• Dynamic Range: the ratio of maximum measurable input amplitude to minimum input amplitude.

Sensor Readout

• Analog-to-Digital converter (ADC) and micro-controller configuration: the simplest readout configuration.
• Data Acquisition Cards (DAQ) connected to a personal computer with USB interface: another readout configuration.
• Smart sensors: have their own digital outputs that make use of communication protocols.

Light Sensors

• Physical principle: light-assisted electron-hole couplet generation.
• Basics: semiconductors have reticular structures with molecules bonded together by sharing electrons.
• Electron-hole couplet generation: when electrons are excited by light, they jump out of the bond, creating free charge and increasing conductivity.
• Doping: introducing atoms with different numbers of electrons to create excess charge.
• Diode: a p-doped and an n-doped semiconductor in contact, creating a depletion region.
• Photodiode: a device that operates by light-assisted electron-hole pair generation, biased with a reverse voltage.

Photodiode Physics

• Photodiode operation: light generates electron-hole couplets in the depleted region, which are accelerated to the contacts, generating a photocurrent.
• Photodiode characteristics: photocurrent increases with light intensity, and has a reverse current direction.
• Dark current: current generated by thermally generated electron-hole couplets, even in the absence of light.

Photodiode Structures

• PN photodiode: a basic structure with a p-doped and an n-doped region.
• PIN photodiode: a structure with an intrinsic region between the p-doped and n-doped regions, providing more depleted region and lower capacitance.
• Light penetration depth: the distance that light can penetrate a silicon material before it is absorbed, dependent on the impinging wavelength.### Sensor Systems

Photodiode Sensitivity and Electrical Model

• Sensitivity of a photodiode: calculated as the ratio between generated photocurrent and optical power density impinging on the sensor
• Responsivity (Rλ): depends on incident light wavelength, applied reverse voltage, and temperature
• Electrical model:
  • Iph: photo-generated current
  • Id: dark current, thermally generated
  • Cj: junction capacitance
  • RSH: shunt resistance
  • Rs: series resistance

Photodiode Noise Model and Readout Circuits

• Noise: statistical variation of the value we want to measure, due to environmental and device-related factors
• Noise models:
  • Thermal (Johnson) noise: due to Brownian motion of free charges
  • Shot noise: due to finite charge of carriers
• Noise Equivalent Power (NEP): amount of optical power impinging on a photodetector to generate a current larger than the noise current

Photodiode Readout Circuits

• Non-inverting configuration:
  • Photodiode current flows into RL, creating a voltage amplified by the OP-AMP
• Transimpedance Amplifier configuration:
  • Photodiode current flows through the feedback, with a fixed voltage across the photodiode
• Charge Amplifier configuration (light integrator):
  • Integrates the photocurrent, with a reset switch that discharges the capacitance

Photodiode Applications

• Cameras: light meters, auto-focus, automatic shutter control
• Medical: X-ray detection, pulse oximeters, blood particle analyzers
• Safety equipment: smoke detectors, flame monitors, security systems
• Automotive: twilight detectors, climate control
• Industry: bar code scanners, brightness controls, rotary encoders, position sensors
• Communications: fiber optic links, optical communications

Other Photodiode Architectures

• Avalanche Photodiode (APD):
  • Internal gain due to avalanche effect
  • Good choice for low signal and high signal-to-noise ratio
• Single-Photon Avalanche Photodiode (SPAD):
  • Applied reverse voltage above breakdown voltage
  • Output is digital (ON or OFF)
• Silicon Photo-Multiplier (SiPM):
  • Array of SPADs in parallel
  • Discrete output proportional to the number of impinging photons

Light Dependent Resistor (LDR)

• Resistance changes in response to light intensity
• Working principle based on photo-conductivity effect
• Applications: cheap light ON/OFF detectors

Image Sensors

• Charge Coupled Devices (CCDs):
  • Pixels based on MOS capacitor
  • Photodiode with a capacitance in series
  • Voltage drop across the insulator proportional to optical intensity
• CMOS active pixel image sensors

Note: I made some minor adjustments to the text, such as removing unnecessary words, rephrasing sentences for better clarity, and using bullet points consistently throughout the summary.### Sensor Systems

CCD Structure

• CCD structure consists of pixels in columns connected and distinguished by active pads
• Rows are divided by channel stops, which are highly doped semiconductors that prevent charge passage between rows
• Each column has one biased and two unbiased electrodes, creating a grid of "Charge Wells"

CCD Readout

• Charge is transferred vertically and then horizontally to read out the charge one pixel at a time
• Three electrodes are needed to avoid charge mixing between pixels

CCD Characterization and Performance

• Four quality parameters: charge generation, charge collection, charge transfer, and charge readout
• Quantum Efficiency (QE) measures the ability of a CCD to collect photons and generate charge
• QE is a probability, expressed as a percentage value, and varies with wavelength
• Fixed Pattern Noise is caused by process non-uniformities and can be calibrated
• Dark Current is generated by thermal effects and is temperature-dependent

CCD Charge Collection

• Well Capacity is the maximum amount of charge that can be held in a pixel
• Saturation occurs when the charge exceeds the well capacity
• Blooming occurs when the charge spills over to other pixels

CCD Charge Transfer

• Charge Transfer Efficiency (CTE) measures the ability to transfer charge from one pixel to another
• Charge Transfer Inefficiency (CTI) is the fraction of electrons deferred by one pixel
• Defects in the silicon crystal lattice can cause inefficiency
• Defects such as Dark Columns, Bright Columns, and Hot Spots can be calibrated

CCD Charge Readout

• Readout noise is the limiting factor in the readout
• Amplifier noise can be reduced by decreasing the circuit bandwidth and increasing the integration time
• Reset noise is caused by voltage fluctuations and can be removed by Correlated Double Sampling

Colour Filtering

• Optical filters are used to discriminate colours
• Demosaicing algorithm is used to fill in missing colour information

Types of CCDs

• Full Frame architecture: entire chip is exposed to light and covered by a mechanical shutter
• Frame Transfer architecture: image array collects charge and stores it in a covered storage array
• Interline Transfer architecture: half of the pixels are used for readout, organized in columns

CCD Applications

• Microscopy and biology
• Astronomy and astro-photography
• Infrared photography

CMOS Active Pixel Sensors

• 3T active pixel structure: photodiode, buffer transistor, row select transistor, and reset transistor
• Electrical model: photodiode generates charge, which is integrated into the stray capacitance
• Readout speed is higher than CCDs, but fill factor is lower

CMOS APS Noise

• Fixed Pattern Noise (FPN) caused by process variations
• Reset noise (KTC noise)
• Amplifier noise
• Shot noise

CMOS Sensor Structure and Readout

• Sensor structure: backside-illuminated technology available
• Readout modes: rolling shutter and global shutter
• Global shutter: entire frame is captured at the same instant

CMOS APS Applications

• High volume imagers for consumer applications
• Imagers for machine vision
• High speed motion capture cameras
• Digital radiography
• Endoscopy

CCD vs. CMOS Image Sensors

• CCD advantages: uniformity, highest fill factor, and higher near-infra-red sensitivity
• CMOS advantages: faster readout, selectable active window, digital output, fewer electronics, and lower power consumption

Temperature Sensors

• Types of temperature sensors: RTDs, thermistors, thermocouples, diode and bandgap temperature sensors, and infrared thermometers

Resistance Temperature Detector (RTD)

• Measures temperature by detecting changes in electrical resistance
• Characterized by nominal resistance R0 and nominal temperature T0
• Material defines its resistivity temperature dependence

Sensor Systems Basics

• A sensor system consists of a sensing part and a processing part, which interacts with a processor (Micro-Controller Unit, MCU) through a single access interface.
• Sensors acquire signals that describe physical phenomena and send the information to the micro-controller.
• A Smart Sensor is a sensor that embeds an Analog-to-Digital converter and a small processing unit that can communicate through simple communication protocols.

Sensor Definitions

• Transducer: a device that transforms energy from one type to another, even if both energy types are in the same domain.
• Sensor: a device that monitors a parameter of a system, without disturbing that parameter.
• Actuator: a component of machines that is responsible for moving or controlling a mechanism or system.

Sensor Characterization

• Sensitivity: the ratio between the change in the output signal to a small change in the input physical signal.
• Resolution (LSB): the smallest increment of measure that a device can make.
• Full Scale Range (FSR): the maximum measurable interval, after which the measure saturates.
• Number of bits (n): in sensors with a digital output, 2n is the number of levels in which the FSR is divided.
• Accuracy: the error between the result of a measurement and the true value.
• Precision (or Repeatability): the sensor's ability to output the same value for the same input over several trials.
• Linearity: the deviation of the output from a best-fit straight line for a given range of the sensor.
• Transfer Function (frequency response): the relationship between the physical input signal and the electrical output signal.
• Bandwidth: the frequency range between the lower and upper cutoff frequencies, within which the sensor transfer function is constant gain or linear.
• Noise: the random fluctuation in the measured value.
• Dynamic Range: the ratio of maximum measurable input amplitude to minimum input amplitude.

Sensor Readout

• Analog-to-Digital converter (ADC) and micro-controller configuration: the simplest readout configuration.
• Data Acquisition Cards (DAQ) connected to a personal computer with USB interface: another readout configuration.
• Smart sensors: have their own digital outputs that make use of communication protocols.

Light Sensors

• Physical principle: light-assisted electron-hole couplet generation.
• Basics: semiconductors have reticular structures with molecules bonded together by sharing electrons.
• Electron-hole couplet generation: when electrons are excited by light, they jump out of the bond, creating free charge and increasing conductivity.
• Doping: introducing atoms with different numbers of electrons to create excess charge.
• Diode: a p-doped and an n-doped semiconductor in contact, creating a depletion region.
• Photodiode: a device that operates by light-assisted electron-hole pair generation, biased with a reverse voltage.

Photodiode Physics

• Photodiode operation: light generates electron-hole couplets in the depleted region, which are accelerated to the contacts, generating a photocurrent.
• Photodiode characteristics: photocurrent increases with light intensity, and has a reverse current direction.
• Dark current: current generated by thermally generated electron-hole couplets, even in the absence of light.

Photodiode Structures

• PN photodiode: a basic structure with a p-doped and an n-doped region.
• PIN photodiode: a structure with an intrinsic region between the p-doped and n-doped regions, providing more depleted region and lower capacitance.
• Light penetration depth: the distance that light can penetrate a silicon material before it is absorbed, dependent on the impinging wavelength.### Sensor Systems

Photodiode Sensitivity and Electrical Model

• Sensitivity of a photodiode: calculated as the ratio between generated photocurrent and optical power density impinging on the sensor
• Responsivity (Rλ): depends on incident light wavelength, applied reverse voltage, and temperature
• Electrical model:
  • Iph: photo-generated current
  • Id: dark current, thermally generated
  • Cj: junction capacitance
  • RSH: shunt resistance
  • Rs: series resistance

Photodiode Noise Model and Readout Circuits

• Noise: statistical variation of the value we want to measure, due to environmental and device-related factors
• Noise models:
  • Thermal (Johnson) noise: due to Brownian motion of free charges
  • Shot noise: due to finite charge of carriers
• Noise Equivalent Power (NEP): amount of optical power impinging on a photodetector to generate a current larger than the noise current

Photodiode Readout Circuits

• Non-inverting configuration:
  • Photodiode current flows into RL, creating a voltage amplified by the OP-AMP
• Transimpedance Amplifier configuration:
  • Photodiode current flows through the feedback, with a fixed voltage across the photodiode
• Charge Amplifier configuration (light integrator):
  • Integrates the photocurrent, with a reset switch that discharges the capacitance

Photodiode Applications

• Cameras: light meters, auto-focus, automatic shutter control
• Medical: X-ray detection, pulse oximeters, blood particle analyzers
• Safety equipment: smoke detectors, flame monitors, security systems
• Automotive: twilight detectors, climate control
• Industry: bar code scanners, brightness controls, rotary encoders, position sensors
• Communications: fiber optic links, optical communications

Other Photodiode Architectures

• Avalanche Photodiode (APD):
  • Internal gain due to avalanche effect
  • Good choice for low signal and high signal-to-noise ratio
• Single-Photon Avalanche Photodiode (SPAD):
  • Applied reverse voltage above breakdown voltage
  • Output is digital (ON or OFF)
• Silicon Photo-Multiplier (SiPM):
  • Array of SPADs in parallel
  • Discrete output proportional to the number of impinging photons

Light Dependent Resistor (LDR)

• Resistance changes in response to light intensity
• Working principle based on photo-conductivity effect
• Applications: cheap light ON/OFF detectors

Image Sensors

• Charge Coupled Devices (CCDs):
  • Pixels based on MOS capacitor
  • Photodiode with a capacitance in series
  • Voltage drop across the insulator proportional to optical intensity
• CMOS active pixel image sensors

Note: I made some minor adjustments to the text, such as removing unnecessary words, rephrasing sentences for better clarity, and using bullet points consistently throughout the summary.### Sensor Systems

CCD Structure

• CCD structure consists of pixels in columns connected and distinguished by active pads
• Rows are divided by channel stops, which are highly doped semiconductors that prevent charge passage between rows
• Each column has one biased and two unbiased electrodes, creating a grid of "Charge Wells"

CCD Readout

• Charge is transferred vertically and then horizontally to read out the charge one pixel at a time
• Three electrodes are needed to avoid charge mixing between pixels

CCD Characterization and Performance

• Four quality parameters: charge generation, charge collection, charge transfer, and charge readout
• Quantum Efficiency (QE) measures the ability of a CCD to collect photons and generate charge
• QE is a probability, expressed as a percentage value, and varies with wavelength
• Fixed Pattern Noise is caused by process non-uniformities and can be calibrated
• Dark Current is generated by thermal effects and is temperature-dependent

CCD Charge Collection

• Well Capacity is the maximum amount of charge that can be held in a pixel
• Saturation occurs when the charge exceeds the well capacity
• Blooming occurs when the charge spills over to other pixels

CCD Charge Transfer

• Charge Transfer Efficiency (CTE) measures the ability to transfer charge from one pixel to another
• Charge Transfer Inefficiency (CTI) is the fraction of electrons deferred by one pixel
• Defects in the silicon crystal lattice can cause inefficiency
• Defects such as Dark Columns, Bright Columns, and Hot Spots can be calibrated

CCD Charge Readout

• Readout noise is the limiting factor in the readout
• Amplifier noise can be reduced by decreasing the circuit bandwidth and increasing the integration time
• Reset noise is caused by voltage fluctuations and can be removed by Correlated Double Sampling

Colour Filtering

• Optical filters are used to discriminate colours
• Demosaicing algorithm is used to fill in missing colour information

Types of CCDs

• Full Frame architecture: entire chip is exposed to light and covered by a mechanical shutter
• Frame Transfer architecture: image array collects charge and stores it in a covered storage array
• Interline Transfer architecture: half of the pixels are used for readout, organized in columns

CCD Applications

• Microscopy and biology
• Astronomy and astro-photography
• Infrared photography

CMOS Active Pixel Sensors

• 3T active pixel structure: photodiode, buffer transistor, row select transistor, and reset transistor
• Electrical model: photodiode generates charge, which is integrated into the stray capacitance
• Readout speed is higher than CCDs, but fill factor is lower

CMOS APS Noise

• Fixed Pattern Noise (FPN) caused by process variations
• Reset noise (KTC noise)
• Amplifier noise
• Shot noise

CMOS Sensor Structure and Readout

• Sensor structure: backside-illuminated technology available
• Readout modes: rolling shutter and global shutter
• Global shutter: entire frame is captured at the same instant

CMOS APS Applications

• High volume imagers for consumer applications
• Imagers for machine vision
• High speed motion capture cameras
• Digital radiography
• Endoscopy

CCD vs. CMOS Image Sensors

• CCD advantages: uniformity, highest fill factor, and higher near-infra-red sensitivity
• CMOS advantages: faster readout, selectable active window, digital output, fewer electronics, and lower power consumption

Temperature Sensors

• Types of temperature sensors: RTDs, thermistors, thermocouples, diode and bandgap temperature sensors, and infrared thermometers

Resistance Temperature Detector (RTD)

• Measures temperature by detecting changes in electrical resistance
• Characterized by nominal resistance R0 and nominal temperature T0
• Material defines its resistivity temperature dependence

This quiz is based on Prof. F. Villa's lectures on Sensor Systems for Electronics Engineering students at Politecnico di Milano. It covers topics related to sensor systems and electronics.