1. Seminar Report
Student name: Tarun Nekkanti
Roll No: 142
In partial fulfillment of the requirements for the award of the degree of
BACHELOR OF ENGINEERING
ELECTRONICS AND COMMUNICATION ENGINEERING
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
MANIPAL INSTITUTE OF TECHNOLOGY
(A Constituent College of Manipal University)
MANIPAL – 576104, KARNATAKA, INDIA
13 October 2010
A sensor; is a device that measures a physical quantity and converts it into a signal which can be
read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts
the measured temperature into expansion and contraction of a liquid which can be read on a
calibrated glass tube. A thermocouple converts temperature to an output voltage which can be
read by a voltmeter. For accuracy, most sensors are calibrated against known standards.
A sensor is a device which receives and responds to a signal. A sensor's sensitivity indicates how
much the sensor's output changes when the measured quantity changes. For instance, if the
mercury in a thermometer moves 1 cm when the temperature changes by 1 °C, the sensitivity is
1 cm/°C (it is basically the slope Dy/Dx assuming a linear characteristic). Sensors that measure
very small changes must have very high sensitivities. Sensors also have an impact on what they
measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the
liquid while the liquid heats the thermometer. Sensors need to be designed to have a small effect
on what is measured; making the sensor smaller often improves this and may introduce other
Characteristics of a good sensor
A good sensor obeys the following rules:
Is sensitive to the measured property
Is insensitive to any other property likely to be encountered in its application
Does not influence the measured property.
Linearity & Sensitivity
Ideal sensors are designed to be linear or linear to some simple mathematical function of the
measurement, typically logarithmic. The output signal of such a sensor is linearly proportional to
the value or simple function of the measured property.
The sensitivity is then defined as the ratio between output signal and measured property. For
example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant
3. with the unit [V/K]; this sensor is linear because the ratio is constant at all points of
The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring.
Often in a digital display, the least significant digit will fluctuate, indicating that changes of that
magnitude are only just resolved. The resolution is related to the precision with which the
measurement is made. For example, a scanning tunneling probe (a fine tip near a surface collects
an electron tunneling current) can resolve atoms and molecules.
Capacitive Displacement Sensors
Capacitive sensors are noncontact devices capable of high-resolution measurement of the
position and/or change of position of any conductive target. The nanometer resolution of high-
performance sensors makes them indispensible in today's nanotechnology world. Capacitive
sensing can also be used to measure the position or other properties of nonconductive targets.
A Capacitive Sensor Measurement System
Capacitive sensor dimensional measurement requires three basic components:
a probe that uses changes in capacitance to sense changes in distance to the target,
driver electronics to convert these changes in capacitance into voltage changes,
a device to indicate and/or record the resulting voltage change.
Each of these components is a critical part in providing reliable, accurate measurements. The
probe geometry, sensing area size, and mechanical construction affect range, accuracy, and
stability. A probe requires a driver to provide the changing electric field that is used to sense the
capacitance. The performance of the driver electronics is a primary factor in determining the
resolution of the system; they must be carefully designed for a high-preformance application.
The voltage measuring device is the final link in the system. Oscilloscopes, voltmeters and data
acquisition systems must be properly selected for the application.
What is Capacitance?
Capacitance describes how the space between two conductors affects an electric field between
them. If two metal plates are placed with a gap between them and a voltage is applied to one of
4. the plates, an electric field will exist between the plates. This electric field is the result of the
difference between electric charges that are stored on the surfaces of the plates. Capacitance
refers to the ―capacity‖ of the two plates to hold this charge. A large capacitance has the capacity
to hold more charge than a small capacitance. The amount of existing charge determines how
much current must be used to change the voltage on the plate. It’s like trying to change the water
level by one inch in a barrel compared to a coffee cup. It takes a lot of water to move the level
one inch in the barrel, but in a coffee cup it takes very little water. The difference is their
When using a capacitive sensor, the sensing surface of the probe is the electrified plate and what
you’re measuring (the target) is the other plate (we’ll talk about measuring non-conductive
targets later). The driver electronics continually change the voltage on the sensing surface. This
is called the excitation voltage. The amount of current required to change the voltage is measured
by the circuit and indicates the amount of capacitance between the probe and the target. Or,
conversely, a fixed amount of current is pumped into and out of the probe and the resulting
voltage change is measured.
How Capacitance Relates to Distance
Capacitance is determined by Area, Gap, and Dielectric (the material in the gap). Capacitance
increases when Area or Dielectric increase, and capacitance decreases when the Gap increases.
The capacitance between two plates is determined by three things:
Size of the plates: capacitance increases as the plate size increases
Gap Size: capacitance decreases as the gap increases
Material between the plates (the dielectric):
Dielectric material will cause the capacitance to increase or decrease depending on the
Area and Dielectric are held constant for ordinary capacitive sensing so only the Gap can change
5. In ordinary capacitive sensing the size of the sensor, the size of the target, and the dielectric
material (air) remain constant. The only variable is the gap size. Based on this assumption, driver
electronics assume that all changes in capacitance are a result of a change in gap size. The
electronics are calibrated to output specific voltage changes for corresponding changes in
capacitance. These voltages are scaled to represent specific changes in gap size. The amount of
voltage change for a given amount of gap change is called the sensitivity. A common sensitivity
setting is 1.0V/100µm. That means that for every 100µm change in the gap, the output voltage
changes exactly 1.0V. With this calibration, a +2V change in the output means that the target has
moved 200µm closer to the probe.
Focusing the Electric Field
Probes use a guard to focus the electric field.
When a voltage is applied to a conductor, an electric field is emitted from every surface. For
accurate gauging, the electric field from a capacitive sensor needs to be contained within the
space between the probe’s sensing area and the target. If the electric field is allowed to spread to
other items or other areas on the target, then a change in the position of the other item will be
measured as a change in the position of the target. To prevent this from happening, a technique
called guarding is used. To create a guarded probe, the back and sides of the sensing area are
surrounded by another conductor that is kept at the same voltage as the sensing area itself. When
the excitation voltage is applied to the sensing area, a separate circuit applies the exact same
voltage to the guard. Because there is no difference in voltage between the sensing area and the
guard, there is no electric field between them to cause current flow. Any conductors beside or
behind the probe form an electric field with the guard instead of the sensing area. Only the
unguarded front of the sensing area is allowed to form an electric field to the target.
Effects of Target Size
The target size is a primary consideration when selecting a probe for a specific application.
When the sensor’s electric field is focused by guarding, it creates a field that is a projection of
the sensor size and shape. The minimum target diameter for standard calibration is 30% of the
diameter of the sensing area. The further the probe is from the target, the larger the minimum
6. Range of Measurement
The range in which a capacitive sensor is useful is a function of the area of the sensing surface.
The greater the area, the larger the range. The driver electronics are designed for a certain
amount of capacitance at the sensor. Therefore, a smaller sensor must be considerably closer to
the target to achieve the desired amount of capacitance. The electronics are adjustable during
calibration, but there is a limit to the range of adjustment.
In general, the maximum gap at which a probe is useful is approximately 40% of the sensing
surface diameter. Standard calibrations usually keep the gap considerably less than that.
Multiple Channel Sensing
Frequently, a target is measured simultaneously by multiple probes. Because the system
measures a changing electric field, the excitation voltage for each probe must be synchronized or
the probes would interfere with each other. If they were not synchronized, one probe would be
trying to increase the electric field while another was trying to decrease it thereby giving a false
Driver electronics can be configured as masters or slaves. The master sets the synchronization
for the slaves in multiple channel systems.
Effects of Target Material
The electric field from the probe sensing area is seeking a conductive surface. For this reason,
capacitive sensors are not affected by the target material provided that it is a conductor. Because
the electric field from the sensor stops at the surface of the conductor, target thickness does not
affect the measurement.
Surface finish can affect the measurement. Capacitive sensors will measure the average position
of the target surface within the spot size of the sensor.
Nonconductors can be measured by passing the electric field through them to a stationary
conductive target behind.
7. Capacitive sensors are most often used to measure the change in position of a conductive target.
But capacitive sensors can be very effective in measuring presence, density, thickness, and
location of non-conductors as well. Non-conductive materials like plastic have a different
dielectric constant than air. The dielectric constant determines how a non-conductive material
affects capacitance between two conductors. By inserting a non-conductive material in the gap
between the probe and a stationary reference target, the capacitance will change in relationship to
the thickness, density, or location of the material.
Fringing can be used to measure nonconductive targets without a conductive background target.
Sometimes it’s not feasible to have a reference target in front of the probe. If the material has a
high dielectric constant and a large sensor is used, measurements can still be made by a
technique called fringing. If there is no conductive surface directly in front of the probe, the
sensor’s electric field will wrap back to the shell of the probe itself. This is called a fringe field.
If a non-conductive material is brought in proximity to the probe, its dielectric will change the
fringe field and this can be used to measure the non-conductive material.
Strategies for maximizing effectiveness and minimizing error
Maximizing Accuracy: Target Size
Small targets make measurement accuracy sensitive to small probe position errors.
Unless otherwise specified, factory calibrations are done with a flat conductive target that is
considerably larger than the sensor area. A system calibrated in this way will give accurate
results when measuring a flat target more than 30% larger than the sensing area. If the target area
is too small, the electric field will begin to wrap around the sides of the target. In this case, the
electric field extends farther than it did in calibration and will measure the target as farther away.
8. This means that the probe must be closer to the target for the same zero point. Because this
distance differs from the original calibration, error will be introduced. Error is also created
because the probe is no longer measuring a flat surface.
An additional problem of an undersized target is that the system becomes sensitive to X and Y
location of the probe relative to the target. Without changing the gap, the output will change
significantly if the probe is moved left or right because less of the electric field is going to the
center of the target and more is going around to the sides.
Maximizing Accuracy: Target Shape
Curved targets change the shape of the electric field, affecting accuracy.
Target shape is also a consideration. Since the capacitive sensors are calibrated to a flat target,
measuring a target with a curved surface will cause errors. Because the sensor will measure the
average distance to the target, the gap at zero volts will be different than when the system was
calibrated. Errors will also be introduced because of the different behavior of the electric field
with the curved surface. In cases where a non-flat target must be measured, the system can be
factory calibrated to the final target shape. Alternatively, when flat calibrations are used with
curved surfaces, multipliers can be provided to correct the measurement value.
Maximizing Accuracy: Surface Finish
Irregular surface finish can cause different measurements as the target moves parallel to the
When the target surface is not perfectly smooth, the capacitive sensor will average over the area
covered by the spot size of the sensor. The measurement value can change as the sensor is moved
across the surface due to a change in the average location of the surface. The magnitude of this
error depends on the nature and symmetry of the surface irregularities.
9. Maximizing Accuracy: Parallelism
During calibration, the surface of the sensor is parallel to the target surface. If the probe or target
is tilted any significant amount, the shape of the spot where the field hits the target elongates and
changes the interaction of the field between the probe and target. Because of the different
behavior of the electric field, measurement errors will be introduced. Parallelism must be
considered when designing a fixture for the measurement.
Parameters used to determine the quality of the sensor
Sensitivity Error - The slope of the actual measurements deviates from the ideal slope.
A sensor’s sensitivity is set during calibration. When sensitivity deviates from the ideal value
this is called sensitivity error, gain error, or scaling error. Since sensitivity is the slope of a line,
sensitivity error is usually presented as a percentage of slope; comparing the ideal slope with the
Offset Error - A constant value is added to all measurements.
Offset error occurs when a constant value is added to the output voltage of the system.
Capacitive sensor systems are usually ―zeroed‖ during setup, eliminating any offset deviations
from the original calibration. However, should the offset error change after the system is zeroed,
error will be introduced into the measurement. Temperature change is the primary factor in offset
10. Linearity Error
Linearity Error - Measurement data is not on a straight line.
Sensitivity can vary slightly between any two points of data. This variation is called linearity
error. The linearity specification is the measurement of how far the output varies from a straight
To calculate the linearity error, calibration data is compared to the straight line that would best fit
the points. This straight reference line is calculated from the calibration data using a technique
called least squares fitting. The amount of error at the point on the calibration curve that is
furthest away from this ideal line is the linearity error. Linearity error is usually expressed in
terms of percent of full scale. If the error at the worst point was 0.001mm and the full scale range
of the calibration was 1mm, the linearity error would be 0.1%.
0.50 -10.000 -9.800 -0.010
0.75 -5.000 -4.900 -0.005
1.00 0.000 0.000 0.000
1.25 5.000 5.000 0.000
1.50 10.000 10.100 0.005
Error Band- the worst case deviation of the measured values from the expected values in a
calibration chart. In this case, the total error is -0.010mm.
11. Error band accounts for the combination of linearity and sensitivity errors. It is the measurement
of the worst case absolute error in the calibrated range. The total error is calculated by comparing
the output voltages at specific gaps to their expected value. The worst case error from this
comparison is listed as the capacitive sensor system’s total error.
High-Performance Capacitive Sensors
It is important to distinguish between "high-performance" sensors and inexpensive sensors.
Simple capacitive sensors, such as those used in inexpensive proximity switches or elevator
touch switches, are simple devices and in their most basic form could be designed in a high
school electronics class. Proximity type sensors are tremendously useful in automation
applications and many commercially available models are well made, but they are not suited to
precision metrology applications.
In contrast, capacitive sensors for use in precision displacement measurement and metrology
applications use complex electronic designs to execute complex mathematical algorithms. Unlike
inexpensive sensors, these high-performance sensors have outputs which are very linear, stable
with temperature, and able to resolve incredibly small changes in capacitance resulting in high
resolution measurements of less than one nanometer.
Compared to other noncontact sensing technologies such as optical, laser, eddy-current, and
inductive, high-performance capacitive sensors have some distinct advantages.
Higher resolutions including sub nanometer resolutions
Not sensitive to material changes: Capacitive sensors respond equally to all conductors
Inexpensive compared to laser interferometers.
Capacitive sensors are not a good choice in these conditions:
Dirty or wet environment (eddy-current sensors are ideal).
Large gap between sensor and target is required (optical and laser are better).
Capacitive sensors are useful in any application requiring the measurement or monitoring of the
position of a conductive target.
12. Capacitive sensors are basically position measuring devices. The outputs
always indicate the size of the gap between the sensor's sensing surface
and the target. When the probe is stationary, any changes in the output
are directly interpreted as changes in position of the target. This is useful
Automation requiring precise location
Final assembly of precision equipment such as disk drives
Precision stage positioning
Measuring the dynamics of a continuously moving target, such as a
rotating spindle or vibrating element, requires some form of noncontact
measurement. Capacitive sensors are ideal when the environment is clean
and the motions are small, requiring high-resolution.
Disk drive spindles
High-speed drill spindles
Measuring material thickness in a noncontact fashion is a common
application for capacitive sensors. The most useful application is a two-
channel differential system in which a separate sensor is used for each
side of the piece being measured. Details on thickness measurements
with capacitive sensors are available in the Conductive Material
Thickness Measurement with Capacitive Sensors Application Note.
Capacitive sensor technology is used for thickness measurement in these
Silicon wafer thickness
Brake rotor thickness
13. Disk drive platter thickness
Capacitive sensors are sensitive to nonconductive materials which are
placed between the probe's sensing area and a grounded back target. If
the gap between the sensor and the back target is stable, changes in the
sensor output are indicative of changes in thickness, density, or
composition of the material in the gap. This is used for measurements in
Label positioning during application
Capacitive sensors have a much higher sensitivity to conductors than to
nonconductors. For this reason, they can be used to detect the
presence/absence of metallic subassemblies in completed assemblies. An
example is a connector assembly requiring an internal metallic snap ring
which is not visible in the final assembly. Online capacitive sensing can
detect the defective part and signal the system to remove it from the line.
Capacitive Sensing in HID
Capacitance sensors detect a change in capacitance when something or someone approaches or
touches the sensor. Capacitive sensing as a human interface device (HID) technology, for
example to replace the computer mouse, is becoming increasingly popular. Capacitive sensors
are used in devices such as laptop track-pads, MP3 players, computer monitors, cell phones and
others. More and more engineers choose capacitive sensors for their flexibility, unique human-
14. device interface and cost reduction over mechanical switches. Capacitive touch sensors have
become a predominant feature in a large number of mobile devices and MP3 players.
Capacitive sensors detect anything which is conductive or having dielectric properties. While
capacitive sensing applications can replace mechanical buttons with capacitive alternatives, other
technologies such as multi-touch and gesture-based touchscreens are also premised on capacitive
A basic sensor includes a receiver and a transmitter, each of which consists of metal traces
formed on layers of a printed-circuit board (PCB). As shown in Figure 1, the AD714x has an on-
chip excitation source, which is connected to the transmitter trace of the sensor. Between the
receiver and the transmitter trace, an electric field is formed. Most of the field is concentrated
between the two layers of the sensor PCB. However, a fringe electric field extends from the
transmitter, out of the PCB, and terminates back at the receiver. The field strength at the receiver
is measured by the on-chip sigma-delta capacitance-to-digital converter. The electrical
environment changes when a human hand invades the fringe field, with a portion of the electric
field being shunted to ground instead of terminating at the receiver. The resultant decrease in
capacitance—on the order of femtofarads as compared to picofarads for the bulk of the electric
field—is detected by the converter.
In general, there are three parts to the capacitance-sensing solutions:
15. The driver IC, which provides the excitation, the capacitance-to-digital converter, and
compensation circuitry to ensure accurate results in all environments.
The sensor—a PCB with a pattern of traces, such as buttons, scroll bars, scroll wheels, or
some combination. The traces can be copper, carbon, or silver, while the PCB can be
FR4, flex, PET, or ITO.
Software on the host microcontroller to implement the serial interface and the device
setup, as well as the interrupt service routine. For high-resolution sensors such as scroll
bars and wheels, the host runs a software algorithm to achieve high resolution output. No
software is required for buttons.
AD714x Driver ICs
These capacitance-to-digital converters are designed specifically for capacitance sensing in
human-interface applications. The core of the devices is a 16-bit sigma-delta capacitance-to-
digital converter (CDC), which converts the capacitive input signals (routed by a switch matrix)
into digital values. The result of the conversion is stored in on-chip registers. The on-chip
excitation source is a 250-kHz square wave.
16. These devices interface with up to 14 external capacitance sensors, arranged as buttons, bars,
wheels, or a combination of sensor types. The external sensors consist of electrodes on a 2- or 4-
layer PCB that interfaces directly with the IC.
The devices can be set up to interface with any set of input sensors by programming the on-chip
registers. The registers can also be programmed to control features such as averaging and offset
adjustment for each of the external sensors. An on-chip sequencer controls how each of the
capacitance inputs is polled.
The AD714x also include on-chip digital logic and 528 words of RAM that are used for
environmental compensation. Humidity, temperature, and other environmental factors can affect
the operation of capacitance sensors; so, transparently to the user, the devices perform
continuous calibration to compensate for these effects, giving error-free results at all times.
One of the key features of the AD714x is sensitivity control, which imparts a different sensitivity
setting to each sensor, controlling how soft or hard the user’s touch must be to activate the
sensor. These independent settings for activation thresholds, which determine when a sensor is
active, are vital when considering the operation of different-size sensors. Take, for example, an
application that has a large, 10-mm-diameter button, and a small, 5-mm-diameter button. The
user expects both to activate with same touch pressure, but capacitance is related to sensor area,
so a smaller sensor needs a harder touch to activate it. The end user should not have to press one
button harder than another for the same effect, so having independent sensitivity settings for each
sensor solves this problem.
Different Shapes & Sizes in capacitance sensing
As noted earlier, the sensor traces can be any number of different shapes and sizes. Buttons,
wheels, scroll-bar, joypad, and touchpad shapes can be laid out as traces on the sensor PCB.
Figure below shows a selection of capacitance sensor layouts.
18. Many options for implementing the user interface are available to the designer, ranging from
simply replacing mechanical buttons with capacitive button sensors to eliminating buttons by
using a joypad with eight output positions, or a scroll wheel that gives 128 output positions.
The number of sensors that can be implemented using a single device depends on the type of
sensors required. The AD7142 has 14 capacitance input pins and 12 conversion channels. The
AD7143 has eight capacitance inputs and eight conversion channels. The table below shows the
number of input pins and conversion stages required for each sensor type. Any number of
sensors can be combined, up to the limit established by the number of available inputs and
Number of CIN inputs
Number of conversion channels
Button 1 1 (0.5 for differential operation)
8-Way Switch 4—top, bottom, left, and right 3
Slider 8—1 per segment 8—1 per segment
Wheel 8—1 per segment 8—1 per segment
1 per row, 1 per column 1 per row, 1 per column
Measurements are taken on all connected sensors sequentially—in a ―round-robin‖ fashion. All
sensors can be measured within 36 ms, though, allowing essentially simultaneous detection of
each sensor’s status—as it would take a very fast user to activate or deactivate a sensor within 40
Capacitance sensors are more reliable than mechanical sensors—for a number of reasons. There
are no moving parts, so there is no wear and tear on the sensor, which is protected by covering
material, for example, the plastic cover of an MP3 player. Humans are never in direct contact
with the sensor, so it can be sealed away from dirt or spillages. This makes capacitance sensors
especially suitable for devices that need to be cleaned regularly—as the sensor will not be
damaged by harsh abrasive cleaning agents—and for hand-held devices, where the likelihood of
accidental spillages (e.g., coffee) is not negligible.
Capacitance sensors are an emerging technology for human-machine interfaces and are rapidly
becoming the preferred technology over a range of different products and devices. Capacitance
sensors enable innovative yet easy-to-use interfaces for a wide range of portable and consumer
products. Easy to design, they use standard PCB manufacturing techniques and are more reliable
than mechanical switches. They give the industrial designer freedom to focus on styling,
knowing that capacitance sensors can be relied upon to give a high-performance interface that
will fit the design.