AN-829
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Environmental Compensation on the AD7142:
The Effects of Temperature and Humidity on Capacitance Sensors
by Susan Pratt
INTRODUCTION
Capacitance sensing has the potential to replace current user
input mechanisms in consumer devices. Products as diverse as
cell phones, digital cameras, MP3 players, and other portable
media players are all suitable for implementing capacitance
sensing.
Capacitance sensing has many benefits. It gives the user an
interface with greater sensitivity and control. Capacitance
sensors are easy to manufacture and reliable, and have
advantages over current mechanical interfaces. However, all
types of capacitance sensors are affected by capacitance changes
in the surrounding environment. Changes in humidity or
temperature can interfere with the operation of the sensors, in
some cases stopping the sensor from working altogether.
Analog Devices’ capacitance sensing solution consists of the
AD7142 capacitive-to-digital converter IC, sensors on the PCB,
and software to communicate with the AD7142. A crucial
element of the AD7142 is the on-chip digital logic that performs
environmental compensation. This ensures that the capacitance
sensor is automatically compensated for the effect of environmental changes. This application note describes the importance
of this on-chip logic by quantifying the effects changing
environmental factors have on typical sensor configurations.
This document also explains how the compensation routines on
the AD7142 ensure that the sensors continue to operate
correctly in changing environmental conditions.
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TABLE OF CONTENTS
Introduction ...................................................................................... 1
Sensor Configurations ..................................................................... 3
Effects of Temperature and Humidity ........................................... 4
Sensor Operation.............................................................................. 5
Environmental Compensation ................................................... 5
REVISION HISTORY
12/05—Revision 0: Initial Version
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SENSOR CONFIGURATIONS
Three types of sensor configuration were tested to quantify the
effect of temperature and humidity changes on the measured
capacitance. The sensor configurations included a joy pad, a
button, and a slider, all connected to the AD7142 as shown in
Figure 1.
SENSOR PCB LAYER 4
SENSOR PCB LAYER 3
30
29
28
27
26
CIN0
VREF –
VREF +
TEST
GPIO
25
INT
31
1
CIN3
CS/ADD1 24
2
CIN4
SCLK 23
3
CIN5
SDI/ADD0 22
4
CIN6
5
CIN7
VDRIVE 20
6
CIN8
DGND2 19
7
CIN9
DGND1 18
8
CIN10
SDO/SDA 21
AD7142
AGND
SRC
SRC
9
10
11
12
13
14
15
16
05848-001
AVCC
DVCC 17
CBIAS
B
CIN13
B
B
CIN12
B
CIN11
SENSOR PCB LAYER 2
SENSOR PCB LAYER 2
SENSOR PCB LAYER 1
SLIDER
JOY
PAD
32
CIN1
SLIDER
CIN2
VCC
Figure 1. Sensor Configurations
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EFFECTS OF TEMPERATURE AND HUMIDITY
31700
Figure 2 through Figure 4 show the changes in the measured
ADC value that occur when the ambient temperature or
humidity in and around the sensors change. The measured
value here is on the ambient capacitance—that is, when the
sensor is not touched.
31600
JOY PAD VALUE (CIN4) 70°C
The effect of an increase in humidity in the ambient environment is also shown in Figure 2 through Figure 4. Again, the
outputs from the sensors change in response to the humidity,
but not in a predictable way.
JOY PAD VALUE (CIN6) 70°C
31300
31200
JOY PAD VALUE (CIN6) 50°C
31000
10
30
50
70
90
05848-003
JOY PAD VALUE (CIN6) 30°C
31100
98
HUMIDITY (%)
Figure 3. Environmental Effects on Joy Pad Sensor
ACTIVATION (CIN1) 30°C
MEASURED VALUE
40700
BUTTON 2 (CIN8) 50°C
51500
51000
BUTTON 2 (CIN8) 30°C
ACTIVATION (CIN1) 10°C
40500
ACTIVATION (CIN1) 50°C
40300
40100
39900
ACTIVATION (CIN1) 70°C
05848-004
39700
50000
39500
49500
BUTTON 1 (CIN7) 30°C
10
BUTTON 1 (CIN7) 50°C
30
50
70
90
HUMIDITY (%)
49000
Figure 4. Environmental Effects on Slider Sensor
48500
05848-002
MEASURED VALUE
JOY PAD VALUE (CIN4) 50°C
31400
40900
52000
BUTTON 2 (CIN8) 70°C
MEASURED VALUE
31500
The figures show that as the ambient temperature increases, the
capacitance measured from each of the sensors changes. (The
measured value is the ADC code output from the capacitive-todigital converter on the AD7142). These changes are not
predictable, because the capacitance measured can increase or
decrease. For example, at 30°C, the Button 1 sensor measures at
roughly 48550. When the temperature increases to 50°C, the
sensor output also increases. However, when the temperature
increases further to 70°C, the output from the sensor drops to
below 48500, which is less than the measured value at 30°C.
50500
JOY PAD VALUE (CIN4) 30°C
BUTTON 1 (CIN7) 70°C
48000
10
30
50
70
90
98
HUMIDITY (%)
Figure 2. Environmental Effect on Button Sensors
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SENSOR OPERATION
52000
When a sensor is touched, the total capacitance associated with
that sensor, and measured by the AD7142, changes. When the
capacitance changes to such an extent that a set threshold is
exceeded, the AD7142 registers this as a sensor activation.
BUTTON 2 VALUE (CIN8)
MEASURED VALUE
51000
For example, consider the case of a button sensor that is
connected to the AD7142. When the button is pressed, the
AD7142 registers an increase in capacitance. If the button is not
pressed, the AD7142 measures the background or ambient
capacitance level. Preprogrammed threshold levels are used to
determine if a change in capacitance is due to a button being
pressed. If the capacitance exceeds the threshold limit, the
AD7142 registers this as a true button press.
BUTTON 2 THRESHOLD
50000
49000
05848-006
BUTTON 1 VALUE (CIN7)
BUTTON 1 THRESHOLD
48000
10
30
50
70
90
98
HUMIDITY (%)
SENSOR
TOUCH
Figure 6. Button Sensors with Fixed Thresholds
The measured capacitance from a sensor may also increase due
to environmental changes. In this case, the sensor may never
register as being activated, as the capacitance value may never
fall below the threshold even when the sensor is pressed.
Figure 5. Sensor Activation
The same principle of thresholds is used to determine if other
types of sensors, such as sliders or joy pads, are being touched.
ENVIRONMENTAL COMPENSATION
Changes in the background capacitance level are generally
much smaller than the changes that occur when a sensor is
touched. However, if the change in measured capacitance due to
the environment is great enough to cross the threshold, the
sensor registers as being touched, when in fact, it was not.
Analog Device’s complete capacitance solution takes the
changing environment into account by actively adapting the
threshold values with the changing environment. This ensures
that the sensors operate correctly in all environments and
prevents false touches or failure to register a valid touch. This
compensation is part of the digital logic on board the AD7142,
and is performed by the part automatically at regular intervals.
Figure 7 shows how the environmental compensation algorithm
changes the threshold levels with changing humidity and thus
ensures that the measured values from the button sensors do
not fall below the threshold value to register a false touch.
In Figure 6, Button 2 threshold is fixed at 51000. When the
measured value from Button 2 falls below this threshold, this
registers as a sensor activation. At low humidity values, the
measured value is well above the threshold, an indication the
button is not touched. However, as the humidity increases, the
value measured from Button 2 decreases and crosses the
threshold at humidity values above 70%. Once the threshold is
crossed, a button press is registered, which in this case is a false
activation. Consider a real-world application in which a you are
listening to an MP3 player. When you leave an air-conditioned
building to go outside on a humid summer day, the MP3 player
could start rewinding a tune midsong. Button 1 is also affected
by the humidity in the same manner.
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51500
BUTTON 2 VALUE (CIN8)
51000
BUTTON 2 THRESHOLD
50500
50000
49500
49000
BUTTON 1 VALUE (CIN7)
48500
48000
05848-007
AMBIENT CAPACITANCE VALUE
05848-005
THRESHOLD
MEASURED VALUE
CDC OUTPUT CODE
65536
BUTTON 1 THRESHOLD
10
30
50
70
90
HUMIDITY (%)
Figure 7. Button Sensors with Adaptive Sensors
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AN-829
Similarly, the threshold values for other sensor types are
adjusted for changing temperature and humidity by the
compensation algorithm. For the slider sensor, shown in
Figure 8, the sensor is registered as being active when the
activation level exceeds either the higher or lower threshold
level. An increase or decrease in ambient temperature could
potentially activate the sensor. However, the adaptive thresholds
ensure that no false touches are registered.
Figure 9 shows how the thresholds governing operation of the
joy pad adapt to changing humidity. The joy pad registers a
touch if the CIN4 measured value exceeds with the left or right
thresholds, or if the CIN6 measured value exceeds either the up
or down thresholds. Again, the environmental compensation
algorithm ensures that the threshold values track the environmental changes and ensures correct operation of the sensor.
41000
32000
40800
RIGHT THRESHOLD
40600
31800
40200
40000
ACTIVATION (CIN1)
39800
LOWER THRESHOLD
39600
39400
JOY PAD VALUE (CIN4)
31600
LEFT THRESHOLD
31400
UP THRESHOLD
05848-008
31200
39200
39000
30
50
31000
JOY PAD VALUE (CIN6)
DOWN THRESHOLD
10
70
TEMPERATURE (°C)
05848-009
MEASURED VALUE
MEASURED VALUE
UPPER THRESHOLD
40400
30
50
70
90
98
HUMIDITY (%)
Figure 8. Slider Sensor with Adaptive Thresholds at Constant 50% Humidity
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Figure 9. Joy Pad Sensor + Thresholds at Constant 30°C.
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NOTES
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NOTES
©2005 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN05848-0-12/05(0)
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