Circuit Note
CN-0095
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
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AD7150
Capacitance to Digital Converter (CDC)
ADP1720
50 mA, High Voltage, Micropower LDO
Using the AD7150 Capacitance-to-Digital Converter (CDC) for Proximity Sensing
Applications
CIRCUIT FUNCTION AND BENEFITS
which may be caused by environmental changes, such as
humidity or temperature, without losing the capability of
proximity sensing.
The circuit described in this document provides the basis for
developing a proximity sensing application using the AD7150
capacitance-to-digital converter (CDC).
CIRCUIT DESCRIPTION
The AD7150 CDC measures the capacitance between two
electrodes and compares its measurement result with a
threshold value, which can be either fixed or dynamically
adjusted by the on-chip adaptive threshold algorithm engine.
A proximity sensing application using AD7150 in the standalone
operation requires very few peripheral components, as shown
in Figure 1. The curcuit needs a supply voltage (Battery B1),
some filtering of the supply voltage (R1, C1), and weak pull-ups
(R2, R3) on the I2C® compatible I/O pins. The red LED, D1,
provides a visual indicator that the AD7150 has detected the
proximity, for example, of a hand. The circuit requires a
capacitive sensing element (SENS1), which can simply consist
of two tracks on an FR4 PCB, as shown in Figure 2.
If the input capacitance is altered, for example, by the presence
of a hand, an output flag is set to indicate that a threshold has
been exceeded, thus indicating proximity.
This on-chip adaptive threshold algorithm engine also enables
the AD7150 to adapt to slow changes in the sensing capacitance,
VDD
VDD
C1
0.1µF
VDD
R2
100kΩ
SENS1
SDA 10
GND
2
VDD
SCL 9
3
CIN2
OUT2 8
4
CIN1
OUT1 7
5
EXC2
EXC1 6
VDD
J1
1
2
3
4
AD7150
1
R3
100kΩ
I 2C
SDA
VDD
GND
SCL
R4
10kΩ
D1
RED
U1
Figure 1. AD7150 as a Proximity Detector in the Standalone Operation
08351-002
B1
3V
R1
+ 220Ω
08351-001
VBAT
Figure 2. AD7150 Proximity Detector Demonstration Board
Rev. 0
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each circuit, and their function and performance have been tested and verified in a lab environment
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©2009 Analog Devices, Inc. All rights reserved.
CN-0095
Circuit Note
COMMON VARIATIONS
The voltage supply circuit shown in Figure 3. uses the ADP1720
LDO (used in the 3.3 V mode) to filter battery noise and to
suppress transient pulses in automotive applications.
Variations in the AD7150 proximity circuit depend on the
environment and the targeted application. For example,
automotive applications must withstand a high level of EMC
noise and transient pulses at the system level. Therefore, this
type of application requires a design suitable for harsh electrical
and physical environments.
If the outputs of the AD7150 are not connected directly to a
microcontroller, they may require conditioning to translate the
voltage level and/or signal polarity. Typical conditioning circuits
for OUT1 and OUT2 are shown in Figure 3. DMOS FETs (Q1
and Q2) act as open drain output drivers, and the 27 V varistors
(V1 and V2) protect the circuitry from large external transients.
The AD7150’s unique design for measuring floating capacitive
sensors allows placing a filter structure in the capacitive frontend. The filter structure (R1 to R6, C1 to C6), as shown in
Figure 3, filters noise coupled into the electrodes of the sensor.
The optional network consisting of R7, R8, C7, and C8 prevents
noise from the external I2C-compatible interface from coupling
back into the circuit.
When connected to a microcontroller, some of the AD7150
registers used by the on-chip adaptive threshold algorithm
engine can be programmed to settings other than the power-up
default settings. This is done via the I2C compatible interface
and enables the AD7150 to be used for different applications
with different requirements. See the AD7150 data sheet for
more details.
Substantial EMC testing has been performed on the AD7150.
The results of the AD7150 EMC performance can be found in
the AN-1011 Application Note, EMC Protection of the AD7150.
Table 1 and Table 2 show a typical proximity performance of the
door handle demonstration with different sensitivity and
capacitive input range settings.
The excitation voltages (EXC1, EXC2), which drive the capactive sensors, are generated by circuits within the AD7150. These
circuits are powered from VDD. Therefore, a noisy supply voltage
can result in unwanted noise signals on the capacitive input.
ADP1720
R16
C12
1µF
220Ω
VOLTAGE SUPPLY
U2
3
OUT
1
GND
7
D1
BAS521
6
100Ω
D2
B2X284-27V
C13
1µF
EN 4
GND
8
R15
IN 2
5
C14
1nF/100V
4
3
2
1
OUTPUTS
CAPACITIVE FRONT END
R4
39kΩ
SENS2
R4
39kΩ
SENS1
VDD
R5
C4 82kΩ
68pF
R6
C5
22pF
10kΩ
C6
47pF
R13
10kΩ
AD7150
R2
C1 82kΩ
68pF
R3
C2
22pF
10kΩ
C3
47pF
Q1
BSS138
LED1
RED
C11
0.1µF
R11
51Ω
J1
POWER/OUT
C9
1nF
V1
27V
C10
1nF
V2
27V
BAT (+)
OUT1
OUT2
GND (–)
SDA 10
1
GND
2
VDD
SCL 9
3
CIN2
OUT2 8
4
CIN1
OUT1 7
5
EXC2
EXC1 6
Q2
BSS138
LED2
ORANGE
U1
R14
10kΩ
VDD
R12
51Ω
I2C-COMPATIBLE INTERFACE
VDD
R9
10kΩ
R10
10kΩ
R7
C7
R8
C8
Figure 3. AD7150 in an Automotive Door Handle Application in the Standalone Operation
Rev. 0 | Page 2 of 4
VDD
J2
I2C
1
2
3
4
SDA
VDD
GND
SCL
08351-003
VDD = 3.3V
Circuit Note
CN-0095
Table 1. Typical Proximity Performance of Sensor 1 on the Door Handle Demonstration Board
2.0 pF
Sensitivity
Setting
4
8
12
16
4
8
12
16
Proximity
Max (mm)
38
31
27
24
25
17
12
10
40
35
MAXIMUM DETECTION (mm)
Cap Range
0.5 pF
0.5pF CAP RANGE
30
25
20
2.0pF CAP RANGE
15
10
0
08351-004
5
4
8
12
16
THRESHOLD SENSITIVITY SETTING
Table 2. Typical Proximity Performance of Sensor 2 on the Door Handle Demonstration Board
2.0 pF
Sensitivity
Setting
4
8
12
16
4
8
12
16
Proximity
Max (mm)
70
58
50
45
45
35
28
24
80
70
MAXIMUM DETECTION (mm)
Cap Range
0.5 pF
60
0.5pF CAP RANGE
50
40
2.0pF CAP RANGE
30
20
0
08351-005
10
4
8
12
16
08351-006
THRESHOLD SENSITIVITY SETTING
Figure 4. AD7150 Door Handle Demonstration Board
The AD7150’s unique design for measuring floating capacitive
sensors makes the AD7150 tolerant of parasitic capacitances to
ground. This allows the use of ground planes to either shield the
capacitive front-end signals from other analog or digital signals
on the board or to shield them from each other. Figure 4 shows
the AD7150 door handle demonstration board where Sensor 2
on the door handle demonstration board has a ground plane on
the entire top layer to prevent proximity detection when a
person leans against the door handle of a car. The sensor
electrodes are placed on the bottom layer in the same way as
shown for Sensor 1. Therefore, Sensor 2 detects proximity only
when a hand reaches behind the door handle.
Rev. 0 | Page 3 of 4
CN-0095
Circuit Note
LEARN MORE
Capacitance-to-Digital Converter (CDC) Technology from
Analog Devices.
AN-1011 Application Note, EMC Protection of the AD7150,
Analog Devices.
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC
Basics, Analog Devices.
MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC
Advanced Concepts and Applications, Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Data Sheets
AD7150 Data Sheet.
ADP1720 Data Sheet.
REVISION HISTORY
7/09—Revision 0: Initial Version
(Continued from first page) "Circuits from the Lab" are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may
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©2009Analog Devices, Inc. All rights reserved. Trademarks and
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CN08351-0-7/09(0)
Rev. 0 | Page 4 of 4