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Engineering Electronic Department
Technical University of Catalonia, UPC Campus Terrassa, SPAIN
Lecturer: Dr. Luis Romeral
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Outline Block 1: Data Acquisition Systems
Subject 1.- TRANSDUCERS SIGNALS AND SIGNALS CONDITIONING. INTRODUCTION
- Data Acquisition System Concept: Block Diagram
- Sensor definition. General Structure of a Sensor
- Sensors Classification
- General Characteristics of the Sensors
- Measurement errors
Subject 2.- REVIEW OF SIGNAL CONDITIONING - Operational Amplifier
- Active and passive transducer connections
- Instrumentation Amplifiers
- Isolated Amplifiers
- Analogue Filters
Subject 3.- AD & DA CONVERSION
- Digital to Analogue Conversion: Parameters
- Analogue to Digital Conversion: types of converters
- AD Converters specifications and errors
- Data Acquisition Systems
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Engineering Electronic Department
Technical University of Catalonia, UPC Campus Terrassa, SPAIN
Lecturer: Dr. Luis Romeral
4
Control System
Device or Equipment dedicated to command the value of a physical
magnitude as a function of a control signal called Reference or Set-
Value
1 4 9
REFERENCE
CONTROLLER POWER
SYSTEM CONTROLLED
PLANT
Output Signals
Energy
Transducers Signal
Conditioning INTERFACES
x(t)
y(t)
Closed Loop
Feed-back signals
Sensor
Sensors are the natural inputs to a measurement system, producing electrical
signals that directly interface to the signal - conditioning element.
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Control System
Device or Equipment dedicated to command the value of a physical
magnitude as a function of a control signal called Reference or Set-
Value
1 4 9
REFERENCE
CONTROLLER POWER
SYSTEM CONTROLLED
PLANT
Output Signals
Energy
Transducers Signal
Conditioning INTERFACES
x(t)
y(t)
Measurements: Log – in
Supervision
Historical Data
Trend Management
Open Loop
Data Acquisition System:
products and / or processes used
to collect information to document
or analyze some phenomenon
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Data Acquisition System Block Diagram
The picture shows the steps needed to take a physical variable and make it usable by a computer
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Data Acquisition System: Smart Sensors
Some current complex sensors accept changing sensor characteristics by programming
internal parameters in a microprocessor built into the sensor. In this way, the usability of
the sensor can be extended to a wide range of applications
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Process of continuous signals
Data Acquisition System
Analogue Digital CPU
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Transducers: convert the physical processes to electrical signals
Examples:
• Temperature
• Pressure
• Light
• Force
Data Acquisition System
• Displacement
• Level
• Electric signals
• ON/OFF switch
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Data Acquisition System
Signal conditioning: Electrical signals are conditioned so they can be used by an analog input
board. The following features may be available:
• Amplification
• Isolation and buffering
• Linearization
• BandwidthLimiting
Analog multiplexer: Allow multiple analog inputs, each with its
own conditioning for different transducers.
The multiplexer channel is selected by the CPU generating an
address on the multiplexer select lines
Sample-and-hold :
The A/D converter requires a small but significant amount of time to convert.
If the analog signal changes during this time, errors may be introduced.
The sample-and-hold reduces these errors by quickly sampling the signal and holding it
steady while the A/D converts it.
Analog to Digital Converter : Converts an analogue signal taken in the plant to a digital value.
Characteristics of the ADC are:
• Input signal type
• Sampling rate
• Throughput
• Resolution
• Range
• Gain
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Sensors: The First Stage in the Measurement Chain
Formula One race cars now have over 150 sensors (exhaust gases, temperature, accelerometers,..)
that communicate via wireless telemetry to the engineering team; these can be sampled up to
1,000 times each second, which can create as much as 2 Mb of data per lap of the race track.
Sensors versus Transducers:
These two terms are regularly interchanged because they identify the same thing — almost.
A transducer changes one form of energy to another, whereas a sensor produces an
electrical output regardless of the energy input. That is, sensors are a subset of transducers.
A voice coil speaker, for example, is a transducer because it converts electrical energy to mechanical
displacement. In comparison, the piezoelectric element found in one type of accelerometer is a
sensor because applied mechanical force creates an electrical output.
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Sensor definition
A sensor is a physical device that detects, or senses, a signal or physical condition.
Sensors are usually paired with an indicator (perhaps indirectly through an analog to
digital converter, a computer and a display) so that the value sensed becomes human
readable or useful for control purposes.
Aside from other applications, sensors are heavily used in industry and aeronautics.
The basic function of an electronic sensor is to measure some feature of the world,
such as light , sound, or pressure and convert that measurement into an electrical
signal, usually a voltage or current.
.
Time
Pre
ssu
re
Time V
olt
ag
e
The sensor converts a physical condition to electrical signal
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General Sensors Types
There are two basic types of sensors: analog and digital.
Analog sensor:
They produce a continuously varying output value over its range of measurement.
For example, a particular photocell might have a resistance of 1k ohm in bright light and a resistance of
300k ohm in complete darkness, and any value between these two is possible.
Digital sensors, on the other hand, have only a finite number of states:
ON - OFF Sensors: The simplest digital sensor has two states, often called "on" and "off,“ and it only
indicates the pass of a certain threshold or limit in the variable.
On/Off sensors are designed for simple, switching applications
DIGITAL Sensors:
These sensors give a signal coded either in form of pulses or in binary code. They produce
pulse trains of transitions between the 0 volt state and the 5 volt state.
With these sensors, the actual element of measuring is an analog device, but signal-processing circuitry
that is integral to the sensor produces a digital output.
The frequency characteristics or shape of this pulse train convey the sensor's measurement.
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General Structure of an Industrial Sensor
- TRANSDUCER: A transducer is a device, usually electrical or electronic, that
converts one type of energy to another. Most transducers give at the output an
electrical signal
- SIGNAL CONDITIONING: Signal conditioning occurs in the interface between
the transducers and the electrical circuit. A low-level signal amplifier and a low-
pass filter are common signal conditioners after the input transducer. The output
signal is usually conditioned by a low-pass filter and some type of power amplifier.
- OUTPUT STAGE: They are switches, power amplifiers, converters or transmitters
that commute or transmit the signal to the external load
Transducer Signal
Conditioning
Output
Amplifier
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Transducers
- Passive Transducers: They require an external electrical source for their
operation.
They are based on the modification of a variable parameter (resistance, capacity or inductance)
that causes changes in the allotment of voltages, which are read by the electronic circuit of
Signal Conditioning.
- Active Transducers: They generate an electrical voltage, based in general on
some of the following priciples:
- Electromagnetism (Electromagnetic induction is the production of an electrical
potential difference (or voltage) across a conductor situated in a changing
magnetic flux
- Piezo - Electricity (Piezoelectricity is the ability of certain crystals to generate a
voltage in response to applied mechanical stress)
- Photo voltaic ( The photoelectric effect is the emission of electrons from matter
upon the absorption of electromagnetic radiation, such as ultraviolet
radiation or x – rays)
- Thermoelectricity (Thermoelectricity is the conversion from temperature
differentials to electricity or vice versa)
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- OUTPUT SIGNAL TYPE
- On-Off
- Digital
- Analogue
Sensors Classification
Some different sensors classifications can be made according the transducer
application, transducer type,.. For instance, we can classify sensors by
- TYPE OF TRANSDUCER
- Electromagnetics
- Ultrasonics
- Resistance,
- Thermoelectricity,.., ...
- APPLICATION
- Presence sensors
- Distance sensors
- Speed Sensors
- Flow sensors,
- Motion sensors
- Acoustic sensors, ....
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Sensors Classification
Since a significant change involves an exchange of energy, sensors can be
classified according to the type of energy transfer that they detect:
- Thermal energy
• temperature sensors: thermocouples, temperature sensitive resistors
(thermistors), bi-metal thermometers and thermostats
• heat sensors: bolometer, calorimeter
- Electromagnetic sensors
• electrical resistance sensors: ohmmeter, multimeter
• electrical current sensors: galvanometer, ammeter
• electrical voltage sensors: leaf electroscope, voltmeter
• electrical power sensors: watt-hour meters
• magnetism sensors: magnetic compass, Hall effect device, magnetometer,
- Mechanical sensors
• pressure sensors: barometer, barograph, pressure gauge, air speed
indicator, rate of climb indicator,
• gas and liquid flow sensors: flow sensor, anemometer, flow meter, gas
meter, water meter, mass flow sensor
• mechanical sensors: position sensor, selsyn, switch, strain gauge
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Sensors Classification
- Chemical sensors
Chemical sensors detect the presence of specific chemicals. Examples are
oxygen sensors, also known as lambda sensors, ion-selective
electrodes, pH glass electrodes, and redox electrodes.
- Optical and radiation sensors
• Electromagnetic time-of-flight. Generate an electromagnetic impulse,
broadcast it, then measure the time a reflected pulse takes to return.
Commonly known as - RADAR (Radio Detection And Ranging)
• Acoustic sensors are a special case in that a pressure transducer is used
to generate a compression wave in a fluid medium (air or water)
• Light time-of-flight. A short pulse of light is emitted and returned by a
retroreflector. The return time of the pulse is proportional to the
distance and is related to atmospheric density in a predictable way.
- Non ionizing radiation
• Light sensors: photocells, photodiodes, phototransistors, photo-electric
tubes, CCDs, Nichols radiometer, Image sensor
• Proximity sensor- A less sophisticated distance sensor. Only detects a
specific proximity. May be optical - combination of a photocell and
LED or laser. May employ a magnet and a Hall effect device.
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Flight Control Equipment:
Sensors/Actuators Products
• Throttle Actuator
• Primary Servoactuators
• Position Sensors
• Acceleration Sensors
• Angular Rate Sensors
20
Sensors/Actuators Products
Sensors (transducers) for applications such as: Turbine Engine Test, Helicopter Health and
Usage Monitoring (HUMS), Ground Vibration Test, Flight Test, Wind Tunnel Test, Fuze Safe,
Low Outgassing, Pyroshock and Force Limited Vibration for spacecraft.
Usually operations are certified to AS9100 and ISO 9001, with calibration procedures
accredited by A2LA to ISO 17025. Products are manufactured to meet specific aerospace
environmental standards, such as RTCA-DO-160 (Environmental Conditions and Test
Procedures for Airborne Equipment) and MIL-STD-810 (Environmental Engineering
Considerations and Laboratory Tests, is a United States Military Standard).
Ground Testing of Aircraft & Defense Vehicles
Sensors for GVT (Ground Vibration Testing), modal
analysis, static load and fatigue testing, reliability and
functional testing, and acoustic testing and certification
Flight Test Sensors for Airborne Application
Sensors for helicopters, aircraft, UAVs and rockets to
measure engine vibration and pressure, flutter, buffeting,
HUMS, rotor track and balance, aerodynamic and inertial
loads, cabin and cockpit noise and ordnance launch
21
The European Aviation Safety Authority (EASA) has been forced to change sensors of
angle of attack of about 3,000 Airbus A320 family and other 740 of the A330-A340, nearly
half of the 7,770 units of the world fleet of those models. It was due to a serious incident
suffered by a Lufthansa A321 on November 5, 2014, while flying from Bilbao to Munich
with 109 people on board.
Example of sensor: Angle of Attack Probe
The Angle of Attack (AoA) Probe provides AoA or
Sideslip (SS) by sensing the direction of local
airflow. It is mounted on the fuselage with the
sensing probe extending through the aircraft
fuselage. The sensing probe moves freely into
the airflow or it is continually driven to null
pressure difference al between the upper and
lower slots in its forward surface. These features
sense the direction of air stream flow (Local AoA
or SS). The angular positon of the sensing probe
is converted to an electrical output by an angular
sensor.
http://www.aerosonic.com/wp-content/uploads/2011/06/Sensors-201106.pdf
22
The Circuit is a Pitot Tube that sticks out into the airstream and senses dynamic pressure.
There is also a static port that senses the pressure outside the aircraft
The airspeed indicator measures the difference between static and dynamic pressure and
displays the result as airspeed. This is called IAS, or Indicated air Speed. The other two
instruments, the Altimeter and the Vertical Speed Indicator, can derive their indications
solely from the static pressure.
Example of sensors: Pitot-Static Circuit
Combined instrument:
pitot tube (right) with a
static port and an
angle-of-attack vane
(left). Air-flow is right
to left.
Note: Look for the Air France Flight n. 447
23
General Characteristics of the Sensors
The response of a real sensor is affected by non ideal behaviors that transform
the response into a curve or families of curves, even for sensors of the same type
and model. Then, a sensor characterization should be made.
The STATIC CHARACTERISTICS describe the performance of the sensor
with very slow/without changes of the variable to measure, under
nominal environmental conditions.
The DYNAMIC CHARACTERISTICS relate the answer of the sensor with
the variations of the magnitude to measure along the time.
The ELECTRIC CHARACTERISTICS define voltages, currents and
impedances of the sensor considered as a black box, from the point of view
of their electric connection to other equipments
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Static Characteristics
SPAN OR RANGE: The smallest and largest values of stimuli the sensor will
encounter. It can be:
Unidirectional:
Zero- fixed : 0 to 2.5 cm.
Shifted: 3 to 8 cm.
Bidirectional:
Symmetrical: -2.5 a 2.5 cm.
Asymmetrical : -2 to 10 cm.
RESOLUTION: The smallest increment of input stimulus that can be sensed
(e.g., the change of a single bit within an analog- to - digital
converter).
It can be given in absolute value of physical variable or per
cent of full scale output
ACCURACY: The deviation of the measured value — the output from the
sensor—from the true value of the measurand.
It is an indication of maximum error, and is given as absolute
value of physical variable or per cent of full scale output
25
Static Characteristics
REPEATABILITY: Repeatability is the variation in measurements obtained
multiple measurements are took using the same sensor
and techniques and in the same environmental conditions
It is an indication of random error and is given as absolute
value of physical variable or per cent of full scale output
0 10 20 30 40 50 60 70 80 90 100
10
20
30
40
50
60
70
80
90
100
Measurement (% range)
Repeatibility (Precision)
Ou
tpu
t (%
EO
S)
26
Static Characteristics
SENSITIVITY: The conversion efficiency of the sensor; the sensor gain of
output amplitude/input amplitude:
The higher the output increment regarding input
increment, the more sensitive the sensor
NOISE: Every value outside the realm of specificity (e.g., shot,
Johnson, or 1/f noise within a device), due to any random
perturbation on the sensor operation
THRESHOLD: The minimum and maximum input detection levels beyond
which the sensor produces no usable output.
SPECIFICITY: Selective conversion of the desired measurand a nd is
relative immunity to other measurands (e.g., a pressure
sensor's ability to reject temperature affects).
)(
)(
physicalVariableInput
electricalVariableOutputySensitivit
27
Static Characteristics
Outp
ut
(%
EO
S
Linearity
LINEARITY: The proportionality of the sensor output to the measured input.
It gives the deviation margins regarding ideal outputs considering
full scale proportional system
It is given as per cent of full scale output
The sensor is linear if the proportionality constant is unique in the
full range of measurement
The Non- linearity gives the maximum deviation regarding ideal
linear characteristic at full scale range.
0 10 20 30 40 50 60 70 80 90 100
10
20
30
40
50
60
70
80
90
100
Measured Variable (% range)
1.1%
1.1%
28
0 10 20 30 40 50 60 70 80 90 100
10
20
30
40
50
60
70
80
90
100
Measured Variable (% range)
Ideal Characteristic
0 10 20 30 40 50 60 70 80 90 100
10
20
30
40
50
60
70
80
90
100
Hysteresis
20 % FSO
Measured Variable (% range)
Hysteresis
HYSTERESIS: The sensor response dependence on previous inputs, the sensor
has a different transfer function for increasing input stimuli from
decreasing input stimuli.
The sensor has hysteresis when for the same input the output differs
depending on the variation of the input, increasing or decreasing.
It is given as absolute value of physical variable or per cent of full
scale output
Static Characteristics
Outp
ut
(% E
OS)
Outp
ut
(% E
OS
)
29
STABILITY: The long-term behavior of the sensor (e.g. temperature drift, or the
change in a pressure sensor's output for changing temperature).
Stability is the ability of a sensor to reproduce output readings
obtained during its original calibration, at room conditions, for a
specified period of time. It is typically expressed as being within X
percent of full scale output or measured units for a period of Y months
Static Characteristics
30
Dynamic Characteristics
FREQUENCY RESPONSE: It is the relationship between the ”GAIN” (Output variable/
input variable) and the FREQUENCY of the input signal,
assuming this is varying sinusoidally.
It is usually given as a graphic response A(db) = f(log F)
Frequency (Hz)
1000
750
500
250
10 100 1K 10K 100K
- 3 dB
Gain
Constant Gain
Bandwidth
31
Dynamic Characteristics
TIME RESPONSE: It is the output sensor evolution regarding time, when the
input is a step signal.
The “Time Constant” is the time required for a sensor
to respond to 63.2% of a stepwise change in a measured
quantity
Initial value
100
90
80
70
60
50
40
30
20
10 5
98
95
63
Time Constant
Response to 95%
Response to 98%
Rising Time
Final Value
Time
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Dynamic Characteristics
TRANSFER FUNCTION: The transfer function T(s) is the ratio of the output function
Vo(s) to the input function Vi (s); where s is the complex
frequency variable.
First Order Systems: The sensor presents a simple delay. Example:
temperature sensors
Second Order Systems: The sensor presents an oscillating response.
Example: mechanical transducer using springs
Examples:
Zero Order: electric switches,
potentiometers,..
First Order: thermocouple,
thermistor, ..
Second Order: springs, ….
High Order: biosensors, semi-
conducting gas sensors,….
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Sortida (% FSO)
Dynamic Characteristics
FIRST ORDER SYSTEMS: Many sensor systems can be modeled with a single time
constant. Although we know that all systems are complex,
it is often possible to get good results with a simplified
model. For example, a system with low-pass behavior may
be accurately modeled with a single response pole,
especially if it is much lower in frequency than other
response elements.
One possible model for a first-order system is
K is a scaling constant and ω0 is the corner
frequency of the single time constant system.
Since the behavior of this system is frequency
dependent, it is useful to determine the magnitude
and phase responses.
A common graphical method used to visualize
these behaviors is the Bode plot, which shows
how transfer functions respond as functions of
frequency. n
34
SECOND ORDER SYSTEMS: Another common physical model contains two energy storage
elements such as capacitance and inductance. Second - order
systems develop a richer variety of response forms; exhibiting
behaviors such as damped sinusoidal oscillations, for example.
The model for one form of second – order system is
As before, K is a scaling constant, ω0 is the undamped
natural frequency of the system, and Q is a quality term
that describes the behavior around ω0.
Users and designers of measurement systems keep these
models in mind to ensure that the frequency response is
appropriate to the physical system that is being measured.
For instance, high - bandwidth signal conditioning
elements are not necessary for low-bandwidth sensors.
Conversely, it is important to have high-bandwidth signal
conditioning for sensors with high bandwidths. Sortida (% FSO)
Dynamic Characteristics
2
21
1log20
nn
jj
n
35
ELECTRICAL CONNECTIONS: The transducer is seen as a “black-box”, without
considering internal operation. Only electrical
interface to others equipments are considered
Input: Gain Stability
Source Impedance Breakdown voltage
Input Impedance Isolation
Output :
Output Impedance
Load Impedance
Output levels (Output Range)
Noise
Electrical Characteristics
Z S
Input
Output Z L
Magnitude Sensor Transducer and electronics
Z ent.
Z sort.
Note: The Output Range determines the
applicability of sensor for data acquisition
equipment
36
Span or range: What are the smallest and largest values of stimuli the sensor reasonably
will encounter)?
You need to determine the useful range of the sensor — does it encompass the anticipated
span of the intended application?
Sensors Characterization
Full scale output: What is the maximum
excursion of the output electrical signal?
That is, what is the difference between the
minimum output for the smallest input
stimulus and the maximum output for the
largest input stimulus?
Accuracy: How much does the measured
value — the output from the sensor —
deviate from the true (NIST - traceable)
value of the measurand?
Does the selected sensor offer the accuracy
required by the application?
(Please note, accuracy is not the same as
resolution!. They are related values but
not synonymous.)
37
Threshold: What are the minimum and maximum input detection levels beyond which the
sensor produces no usable output? That is, if your measurand varies somewhat above or
below estimated nominal values, will you still be able to measure it?
Resolution: What is the smallest increment of input stimulus that can be sensed? The
smallest increment sensed is not necessarily the accuracy because the transfer function
may be nonlinear
Sensors Characterization
Linearity: What is the form of the transfer
function relationship between measurand
input and sensor output?
A linear relationship means that it is very
simple to convert sensor output to final
measurement result.
However even for highly nonlinear sensors,
this usually will not be a concern because
of computing horsepower available for
linearization somewhere along the chain of
measurements.
38
Sensors Characterization
Precision: How repeatable are the measurements from the sensor?
That is, for a measurement repeated with identical input conditions, how much will the
results vary and how much can you tolerate?
Please note, precision is not accuracy:
Sensitivity: What is the conversion efficiency of the sensor? Think of this as the
sensor gain: out/in.
This parameter will affect subsequent signal processing steps and contributes to
overall SNR.
Specificity: Does the sensor offer a highly selective conversion of the desired
measurand, which is relatively immune to others? Temperature effects are ubiquitous.
You may not want to measure temperature with your sensor, but it is likely to be
influenced—sometimes strongly—by temperature. Many sensors include some form of
temperature compensation to minimize such unintended measurand effects;
alternatively, you may need to perform this yourself.
- Accuracy describes how close the sensor is to a static ideal.
- Precision describes how results vary dynamically; i.e., for identical input, how close
the output returns to the same value
39
Hysteresis: Does the sensor response depend on previous inputs? For example, will a
sensor provide the same result for a pressure of 1,000 kPa regardless of whether it was
raised from 500 kPa to the target value or was reduced from 1,500 to that level?
Sensors Characterization
Stability: Is the long-term behavior of the
sensor adequate for the application?
If the sensor is installed today, will it give
acceptable performance next year—or at
least until the next calibration cycle?
Survivability: This is a statement of
ruggedness, environmental suitability, etc.
Can the fundamental sensor element in
combination with its packaging and
interconnect survive in the environment
of the measurand?
Safety: Does the sensor offer intrinsic
safety compatible with the application
environment?
40
Accuracy
The accuracy of an analytical measurement is how close a result comes to the
true value. Determining the accuracy of a measurement usually requires
calibration of the analytical method with a known standard.
Precision
Precision is the reproducibility of multiple measurements and is usually
described by the standard deviation, standard error, or confidence interval.
Accuracy and Precission
41
A good sensor applies to the following rules:
• the sensor should be sensitive to the measured property
• the sensor should be insensitive to any other property
• the sensor should not influence the measured property
In the ideal situation, the output signal of a sensor is exactly proportional to the value of the
measured property. The gain is then defined as the ratio between output signal and measured
property.
If the sensor is not ideal, several types of deviations can be observed:
• The gain may in practice differ from the value specified. This is called a gain error.
• Since the range of the output signal is always limited, the output signal will eventually
clip when the measured property exceeds the limits. The full scale range defines the
outmost values of the measured property where the sensor errors are within the
specified range.
• If the output signal is not zero when the measured property is zero, the sensor has an
offset or bias. This is defined as the output of the sensor at zero input.
• If the gain is not constant, this is called nonlinearity. Usually this is defined by the
amount the output differs from ideal behaviour over the full range of the sensor, often
noted as a percentage of the full range.
Measurement errors
42
• If the deviation is caused by a rapid change of the measured property over time, there is a
dynamic error. Often, this behavior is described with a bode plot showing gain error and
phase shift as function of the frequency of a periodic input signal.
If the signal is monitored digitally, limitation of the sampling frequency also causes a
dynamic error
• If the output signal slowly changes independent of the measured property, this is defined
as drift. Long term drift usually indicates a slow degradation of sensor properties over a
long period of time.
• Noise is a random deviation of the signal that varies in time.
• Hysteresis is an error caused by the fact that the sensor not instantly follows the change of
the property being measured, and therefore involves the history of the measured property.
• If the sensor has a digital output, the signal is discrete and is essentially an approximation
of the measured property. The approximation error is also called digitization error.
The sensor may to some extent be sensitive for other properties than the property being measured.
For example, most sensors are influenced by the temperature of their environment.
All these deviations can be classified as systematic errors or random errors. Systematic errors can
sometimes be compensated for by means of some kind of calibration strategy.
Noise is a random error that can be reduced by signal processing, such as filtering, usually at the
expense of the dynamic behaviour of the sensor.
Measurement errors
43
Sensors’ operations: Range
Sensor operations must take into account the Range of the sensor
44
There are more ranges to take into account in a sensing system, and operations should
be related to all of them.:
Sensors’ operations: Range
45 Solution: shielded measurements, isolation systems ...
Sensors’ operations: Interferences (electro-magnetic noise)
46
Active and passive analogue transducers
Sensor
Electronics +
Vin Vout
+
VS
ZS IS ZS
PASSIVE
ACTIVE
They need external a power supply sensor to produce a voltage or current output
They generate its own voltage or current output
