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CAPACITIVE SENSING
Presented by Alessandro Vieira, Zhan Fang and Yi-Wei
Lu
CAPACITIVE SENSING
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Introduction – Team and Outline
Section 1 – Cap Sense Basic (Alessandro Vieira)
What is Cap Sense?
How does it work?
Section 2 – Cap Sense Applications (Alessandro Vieira)
What is it good for?
Example: Motion Detection
Example: Cap Sense Touch Screen Applications
Section 3 – Measurement Methods. (Zhan Fang)
Basic Idea and Charge Transfer
CSR, CSA, CSD
Section 4 – Cap Sense Challenges (Yi-Wei Lu)
Tuning
Manufacturing Fine Tuning
Conclusion (Yi-Wei Lu)
Oscillator
V REF
8-bit
EN PWM
EN
16-bit
Timer
Data
Processing
SW 1
CX
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2100
2090
Raw Count
2080
2070
Baseline
2060
Finger Threshold
2050
Positive Noise
Threshold
Negative Noise
Threshold
2040
2030
2020
2010
0
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500
1000
1500
2000
2500
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CAPACITIVE SENSING
SECTION 1 – CAP SENSE BASICS
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SECTION 1 – CAP SENSE BASICS
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What is Capacitive Sensing?
It is simply a detection of the presence / absence of
conductive objects.
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Example:
Fingers touching a device
and the added capacitance from the finger is detected.
A Common application this touch capacitive sensing is the
simulation of buttons, sliders, touch pad and even the
ability to do proximity sensing.
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SECTION 1 – CAP SENSE BASICS
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How does it work?
Using touch sensing as an example, the cross-sectional
view of a PCB is shown below.
Sensor capacitor is placed under a non-conductive surface
(ie: glass)
Capacitance exists from sensor to copper ground (Cp)
An electric field is generated
The picture below shows when no finger is present.
Capacitive Sensor
Ground Plane
Ground Plane
E-Field Lines
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SECTION 1 – CAP SENSE BASICS
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How does it work (continues…)?
With the presence of a finger, additional capacitance
(Cf) is added to the sensor
Total sensor capacitance now becomes:
Cx = Cp + Cf
and E-field increases

If we can some how
measure the change
of the capacitance,
we can determine
presence of the Ground Plane
finger.
Button
E-Field Lines
Ground Plane
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CAPACITIVE SENSING
SECTION 2 – APPLICATIONS
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Section 2: Cap Sense Applications
Cap Sense Touch Screen
• Mobile, tablets, LCD, HMI
Others
• Laptop mouse, key switch, sliders,
keypads, etc.
Industrial Sensors
• Material detection, Flow,
Pressure,
Temperature, Accelerometers, Ice
detection, Non-invasive level
detection, Proximity detection, etc
Example 1: Proximity Detection for Human-Friendly
Robots
Example 1: Proximity Detection for Human-Friendly
Robots
Without shield, the e-field is mostly distributed
between
The electrode and the robot.
A driven shield directs the e-field from the
electrode towards the object.
• Increase the proximity sensor range.
• Eliminate crosstalk.
• Reduce noise.
Example 1: Proximity Detection for Human-Friendly
Robots
Reference plate
Insulation
CRobot
Ca
Cm
Cr
Cb
Shield plate
Measuring plate
Vm  Cm  (Vm  Vr )  Cr  (Vm  Vs )  Cb  0
Vm  Vs
Vm 
Vide
o
Vr  Cr
Cm  Cr
Example 2: Touch
Screen
Example 2: Touch
Screen
Applications:
• Mobile, tablets, monitors
Advantages:
• Multi-Touch sensitive
• Scratch resistance
• Can be used through a glass window.
• Immune to dirt environment
• Long lasting (Wear resistant)
• Unaffected by moisture, rain or
temperature
• Can be used with a finger
• Does not requires periodic recalibration
• Low power consumption
• Can be used on curved surfaces (flex)
• Can be used on domed surfaces
• High resolution
• Touch coordinates are drift free
• Can be used with a gloved hand
• Scalable to larger screens
• Can be used with LCD monitor
• Allows hovering sensing
• Lower processing power when compared
to image processing devices
• Replace mechanical buttons in monitors
allowing for a more flexible design.
Disadvantages:
• Touch detection does not work when further away
from screen .
CAPACITIVE SENSING
SECTION 3 – MEASUREMENT METHODS
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SECTION 3 – MEASUREMENT METHODS
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For the rest of this section, we will focus on measurement
methods for capacitive touch sensing
Today, capacitive touch sensing measurements are
typically done with the use of MCUs (mixed-signal
components).
Highly Integrated
Cost Effective
Small Form Factor

A list of some of the cap sense measurement
methods:
Relaxation Oscillator (CSR)
Successive Approximation (CSA)
Sigma-Delta Modulator (CSD)
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SECTION 3 – MEASUREMENT METHODS
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Basic Idea
Uses the RC network to charge (or discharge) circuit voltage to a
reference level.
When a finger is present, the sense capacitance will increase.
Therefore, the time needed to reach the reference level will be
significantly different from the one without the added capacitance.
Successful detection of this difference will indicate the presence of a
finger.
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SECTION 3 – MEASUREMENT METHODS
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Charge Transfer
Uses the charges from the sensing capacitor and transfer them into
a larger integrating capacitor over many switch cycles to build up
the voltage level for comparison.
The additional sensing capacitance allows more charges to be
stored in the sense cap and thus a faster overall transfer (less time
count).
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SECTION 3 – MEASUREMENT METHODS
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Charge Transfer Method – continues…
The voltage across the integrating capacitor keeps stepping up at
each switch cycle. At the same time, the voltage is compared
against a voltage reference. When it reaches the reference
level, the number of switch cycles are recorded.
With the presence of a finger
=> higher Cx
=> more charge transfer during each switch cycle
=> faster voltage rise time
=> significant number of less cycle count
=> additional processing to determine detection
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SECTION 3 – MEASUREMENT METHODS
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Relaxation Oscillator Method (CSR)
Uses the sensing capacitor as a timing element to change the
frequency of an oscillator.
The additional sensing capacitance increases the RC time constant
and thereby, reduces the output frequency of the detection circuit.
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SECTION 3 – MEASUREMENT METHODS
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Relaxation Oscillator Method (CSR) - continues…
The IDAC current source gives charges to Cx and brings up the
voltage to Vref. When the comparator detects this, it will change
the output.
The latched signal then closes the switch to start the discharging
event. The process is repeated to create an oscillation.
As Cx increase:
=> charging/discharging becomes slower
=> oscillation frequency becomes slower
=> timer count goes higher
=> indicates presence of a finger
O scillator
V REF
8-bit
E N PW M
EN
16-bit
Tim er
D ata
Processing
SW 1
CX
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SECTION 3 – MEASUREMENT METHODS
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Successive Approximation Method (CSA)
This method uses varying currents applied to the capacitor
network to sensitize a change in voltage ramp when the sense
cap detects a finger.
The difference is then match against a reference voltage ramp to
determine the activation state and level.
iDAC
↓
(Caps) Voltage => Cycle Counts => Decision
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SECTION 3 – MEASUREMENT METHODS
•
Sigma-Delta Modulator Method (CSD)
This method reads the duty cycle of a modulated output
waveform to determine if a finger is absent / present.
The presence of a finger increases sensed capacitance and in turn
causes the voltage comparison result to stay longer as a ‘1’ in the
cycle.
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SECTION 3 – MEASUREMENT METHODS
•
Sigma-Delta Modulator Method (CSD) – continues…
Additional sense capacitance (Cx):
=> faster and stronger charge of comparator input
=> once the comparator is tripped, discharge through Rb starts,
=> but goes slower because of stronger charging still happening
=> the encoded bit stream will have more 1s
=> longer duty cycle is recognized
V DD
16-bit
PRS
Oscillator
1/2
SW 1
ADCPW M
External
Components
EN
V REF
SW 2
C MOD
16-bit
Timer
Data
Processing
RB
MOD
CX
SW 3
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CAPACITIVE SENSING
SECTION 4 – CAP SENSE CHALLENGES
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SECTION 4 – CAP SENSE CHALLENGES
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SECTION 4 – CAP SENSE CHALLENGES
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Tuning
 Noise
 Signal to Noise Ratio
 Sensor Scan Time
 Finger Threshold (wet finger)
Production Fine Tuning
 PCB manufactures
 Overlay Thickness
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SECTION 4 – CAP SENSE CHALLENGES
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Tuning  Noise
 LCD Inverters
 Signal Cross talk
 RF Interference
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SECTION 4 – CAP SENSE CHALLENGES
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Tuning  Signal to Noise Ratio
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SECTION 4 – CAP SENSE CHALLENGES
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Tuning  Sensor Scan Time
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SECTION 4 – CAP SENSE CHALLENGES
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Tuning  Finger Threshold
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SECTION 4 – CAP SENSE CHALLENGES
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Production Fine Tuning  Difference in PCB
Manufacture
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SECTION 4 – CAP SENSE CHALLENGES
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Production Fine Tuning  Difference in Overlay
Thickness
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SECTION 4 – CAP SENSE CHALLENGES
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Production Fine Tuning  Difference in Overlay
Thickness
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SECTION 4 – CAP SENSE CHALLENGES FIX
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Production Fine Tuning & Tuning is Time
consuming
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Design a circuit that is robust enough  Layout
Example: LCD Touch Sense Buttons
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SECTION 4 – CAP SENSE CHALLENGES FIX
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Production Fine Tuning & Tuning is Time
consuming
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Design a circuit that is robust enough  Layout
Example: LCD Touch Sense Buttons
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SECTION 4 – CAP SENSE CHALLENGES FIX
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Production Fine Tuning & Tuning is Time
consuming
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Auto Tuning
Design a Circuit that is a test bed which can measure all
the parameters that effects SNR:
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
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Sensor Scan Time
Temperature
Parasitic
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SECTION 4 – CAP SENSE CHALLENGES FIX
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Production Fine Tuning & Tuning is Time
consuming

Auto Tuning
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CAPACITIVE SENSING – CONCLUSION
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Described what Cap Sense is and its basics.
Introduced some of the cap sense applications such
as motion detector and touch screen.
Introduced some of the Cap sense measurement
methods using components inside a MCU.
Discussed some of the challenges in the cap sense
technology
Thank you for your time. Questions?...
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APPENDIX A – LINKS TO REFERENCES
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Cypress Touch Sensing Videos
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Cypress Cap Sense FAQ
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Pictures from Google
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Alessandro’s 2001 Motion Sense Project Youtube Video
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Proximity Sense from Freescale
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Overview of Capacitive Sensors
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Thermostat That Uses Capacitive Sensing
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Thermometry Introduction
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Designing A Capacitive Sensing System For A Specific
Application
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Capacitive Sensing Techniques and Considerations
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Multitouch Technologies
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PCB-Based Capacitive Touch Sensing With MSP430
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Button, Slider & Wheel Solutions from Atmel
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Capacitive Sensing Solutions from Silicon Labs
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APPENDIX B – PROXIMITY DETECTION FOR HUMAN-FRIENDLY
ROBOTS
ETHZ Cap Sense Research
Project
APPENDIX C – METHOD SELECTION
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How do we choose the methods?
Engineers must understand the many ways the underlying technology can be
manipulated to improve sensitivity, accuracy, responsiveness and noise immunity of
the capacitive sensing systems.
Method selections are based on the actual application requirements versus the
advantages / limitations of the methods and design.
Charge Transfer, CSA and CSD are some of the most popular methods used today for
touch sensing.
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Charge Transfer:
This sensing method is fairly robust and has a comparatively high SNR.
However, this method requires a precision reference voltage and usually it must
be created with external components (resistors).
Additionally, it requires high-quality dedicated voltage regulators to filter out
noise coming from the power supply for better noise immunity.
The implementation requires multiplexing circuitry within the design. The lack of
such circuitry means additional pins, resistors and capacitors are required for sensing
and tuning of sensitivity for each sensor element.
from Capacitive Sensing Techniques and Considerations
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APPENDIX C – METHOD SELECTION (CONTINUES)
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CSA:
Requires the fewest number of external components; a
single capacitor is used in most applications and may not be
necessary depending on the application needs. The internal
capacitance from the iDAC and sensor capacitor interconnect
may be enough to achieve the needed level of noise immunity.
Requires less power to run in comparing to the CSD method.
Therefore, it helps to maximize battery life and lower operating
voltage (ie: to 2.7V).
Additionally, this method has better immunity to power
supply noises because the switched-capacitor is an equivalent
resistor to ground and the iDAC is sourced from internal
references, not directly by VDD.
from Capacitive Sensing Techniques and Considerations
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APPENDIX C – METHOD SELECTION (CONTINUES…)
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CSD:
This method gives a better immunity to EMI and radiated
emissions of the cap sensing circuits.
It is using a pseudo-random control of the switched-capacitor
network, pseudo-random sequence generator (PRS) – rather than a
fixed-frequency clock – can be used to clock the switched-capacitor
network.
However, the PRS demands a longer scanning time necessary for
the individual sensors, which making this method not suitable for
high-speed applications.
If it is more important to have CapSense plus many other features,
high noise immunity and a thick overlay, then the Sigma-Delta
Modulation method is the best choice.
It is particularly useful in automotive, white goods and industrial
applications.
from Capacitive Sensing Techniques and Considerations
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