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AD7142 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer AD7142
Beschreibung Programmable Controller
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 30 Seiten
AD7142 Datasheet, Funktion
FEATURES
Programmable capacitance-to-digital converter
36 ms update rate (@ maximum sequence length)
Better than 1 fF resolution
14 capacitance sensor input channels
No external RC tuning components required
Automatic conversion sequencer
On-chip automatic calibration logic
Automatic compensation for environmental changes
Automatic adaptive threshold and sensitivity levels
On-chip RAM to store calibration data
SPI®-compatible serial interface (AD7142)
I2C®-compatible serial interface (AD7142-1)
Separate VDRIVE level for serial interface
Interrupt output and GPIO
32-lead, 5 mm x 5 mm LFCSP_VQ
2.6 V to 3.6 V supply voltage
Low operating current
Full power mode: less than 1 mA
Low power mode: 50 μA
APPLICATIONS
Personal music and multimedia players
Cell phones
Digital still cameras
Smart hand-held devices
Television, A/V, and remote controls
Gaming consoles
GENERAL DESCRIPTION
The AD7142 and AD7142-1 are integrated capacitance-to-
digital converters (CDCs) with on-chip environmental
calibration for use in systems requiring a novel user input
method. The AD7142 and AD7142-1 can interface to external
capacitance sensors implementing functions such as capacitive
buttons, scroll bars, or wheels.
The CDC has 14 inputs channeled through a switch matrix to a
16-bit, 250 kHz sigma-delta (∑-Δ) capacitance-to-digital
converter. The CDC is capable of sensing changes in the
capacitance of the external sensors and uses this information to
register a sensor activation. The external sensors can be
arranged as a series of buttons, as a scroll bar or wheel, or as a
combination of sensor types. By programming the registers, the
user has full control over the CDC setup. High resolution
sensors require minor software to run on the host processor.
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarksandregisteredtrademarksarethepropertyoftheirrespectiveowners.
Programmable Controller for
Capacitance Touch Sensors
AD7142
CIN0
CIN1
CIN2
CIN3
CIN4
CIN5
CIN6
CIN7
CIN8
CIN9
CIN10
CIN11
CIN12
CIN13
30
31
32
1
2
3
4
5
6
7
8
9
10
11
FUNCTIONAL BLOCK DIAGRAM
VREF– VREF+
29 28
TEST
27
POWER-ON
RESET
LOGIC
16-BIT
Σ-Δ
CDC
CALIBRATION
ENGINE
CONTROL
AND
DATA
REGISTERS
CALIBRATION
RAM
13 AVCC
14 AGND
17 DVCC
18 DGND1
19 DGND2
CSHIELD 12
SRC 15
SRC 16
250kHz
EXCITATION
SOURCE
VDRIVE 20
SERIAL INTERFACE
AND CONTROL LOGIC
INTERRUPT
AND GPIO
LOGIC
26 GPIO
21
SDO/
SDA
22 23 24
SDI/
ADD0
SCLK
CS/
ADD1
Figure 1.
25
INT
The AD7142 and AD7142-1 have on-chip calibration logic to
account for changes in the ambient environment. The calibration
sequence is performed automatically and at continuous intervals,
when the sensors are not touched. This ensures that there are no
false or nonregistering touches on the external sensors due to a
changing environment.
The AD7142 has an SPI-compatible serial interface, and the
AD7142-1 has an I2C-compatible serial interface. Both parts
have an interrupt output, as well as a general-purpose input/
output (GPIO).
The AD7142 and AD7142-1 are available in a 32-lead, 5 mm ×
5 mm LFCSP_VQ and operate from a 2.6 V to 3.6 V supply. The
operating current consumption is less than 1 mA, falling to
50 μA in low power mode (conversion interval of 400 ms).
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2007 Analog Devices, Inc. All rights reserved.






AD7142 Datasheet, Funktion
AD7142
SPI TIMING SPECIFICATIONS (AD7142)
TA = −40°C to +85°C; VDRIVE = 1.65 V to 3.6 V; AVCC, DVCC = 2.6 V to 3.6 V, unless otherwise noted. Sample tested at 25°C to ensure
compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of VCC) and timed from a voltage level of 1.6 V.
Table 4. SPI Timing Specifications
Parameter
Limit at TMIN, TMAX
fSCLK
5
t1 5
t2 20
t3 20
t4 15
t5 15
t6 20
t7 16
t8 15
Unit
MHz max
ns min
ns min
ns min
ns min
ns min
ns max
ns max
ns min
Description
CS falling edge to first SCLK falling edge
SCLK high pulse width
SCLK low pulse width
SDI setup time
SDI hold time
SDO access time after SCLK falling edge
CS rising edge to SDO high impedance
SCLK rising edge to CS high
CS
SCLK
SDI
SDO
t1 t2
1
t4
t5
MSB
t3
2
3
15 16
1
t8
2 15 16
LSB
t6
MSB
Figure 3. SPI Detailed Timing Diagram
t7
LSB
Rev. A | Page 6 of 72

6 Page









AD7142 pdf, datenblatt
AD7142
THEORY OF OPERATION
The AD7142 and AD7142-1 are capacitance-to-digital
converters (CDCs) with on-chip environmental compensation,
intended for use in portable systems requiring high resolution
user input. The internal circuitry consists of a 16-bit, ∑-Δ con-
verter that converts a capacitive input signal into a digital value.
There are 14 input pins on the AD7142 and AD7142-1, CIN0 to
CIN13. A switch matrix routes the input signals to the CDC.
The result of each capacitance-to-digital conversion is stored in
on-chip registers. The host subsequently reads the results over
the serial interface. The AD7142 contains an SPI interface and
the AD7142-1 has an I2C interface ensuring that the parts are
compatible with a wide range of host processors. Because the
AD7142 and AD7142-1 are identical parts, with the exception of
the serial interface, AD7142 refers to both the AD7142 and
AD7142-1 throughout this data sheet.
The AD7142 interfaces with up to 14 external capacitance
sensors. These sensors can be arranged as buttons, scroll bars,
wheels, or as a combination of sensor types. The external
sensors consist of electrodes on a single or multiple layer PCB
that interfaces directly to the AD7142.
The AD7142 can be set up to implement any set of input
sensors by programming the on-chip registers. The registers can
also be programmed to control features such as averaging,
offsets, and gains for each of the external sensors. There is a
sequencer on-chip to control how each of the capacitance
inputs is polled.
The AD7142 has on-chip digital logic and 528 words of RAM
that are used for environmental compensation. The effects of
humidity, temperature, and other environmental factors can
effect the operation of capacitance sensors. Transparent to the
user, the AD7142 performs continuous calibration to compen-
sate for these effects, allowing the AD7142 to give error-free
results at all times.
The AD7142 requires some minor companion software that
runs on the host or other microcontroller to implement high
resolution sensor functions such as a scroll bar or wheel.
However, no companion software is required to implement
buttons, including 8-way button functionality. Button sensors
are implemented completely in digital logic on-chip.
The AD7142 can be programmed to operate in either full power
mode, or in low power automatic wake-up mode. The
automatic wake-up mode is particularly suited for portable
devices that require low power operation giving the user
significant power savings coupled with full functionality.
The AD7142 has an interrupt output, INT, to indicate when
new data has been placed into the registers. INT is used to
interrupt the host on sensor activation. The AD7142 operates
from a 2.6 V to 3.6 V supply, and is available in a 32-lead, 5 mm ×
5 mm LFCSP_VQ.
CAPACITANCE SENSING THEORY
The AD7142 uses a method of sensing capacitance known as
the shunt method. Using this method, an excitation source is
connected to a transmitter generating an electric field to a
receiver. The field lines measured at the receiver are translated
into the digital domain by a ∑-Δ converter. When a finger, or
other grounded object, interferes with the electric field, some of
the field lines are shunted to ground and do not reach the
receiver (see Figure 18). Therefore, the total capacitance
measured at the receiver decreases when an object comes close
to the induced field.
PLASTIC COVER
Rx
PCB LAYER 1
Tx
AD7142
Σ-Δ
ADC
16-BIT
DATA
EXCITATION
SIGNAL
250kHz
Figure 18. Sensing Capacitance Method
In practice, the excitation source and ∑-Δ ADC are implemented
on the AD7142, and the transmitter and receiver are constructed
on a PCB that makes up the external sensor.
Registering a Sensor Activation
When a sensor is approached, the total capacitance associated
with that sensor, 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 touch.
Preprogrammed threshold levels are used to determine if a
change in capacitance is due to a button being activated. If the
capacitance exceeds one of the threshold limits, the AD7142
registers this as a true button activation. The same thresholds
principle is used to determine if other types of sensors, such as
sliders or scroll wheels, are activated.
Rev. A | Page 12 of 72

12 Page





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