AD7142 Analog Devices, AD7142 Datasheet - Page 13

• Manufacturer Part Number: AD7142
• Description: Programmable Controller for Capacitance Touch Sensors
• Manufacturer: Analog Devices

Specifications of AD7142

• Resolution (bits): 16bit
• # Chan: 14
• Sample Rate: 250kSPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: ± 2 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

Complete Solution for Capacitance Sensing
Analog Devices, Inc. provides a complete solution for
capacitance sensing. The two main elements to the solution are
the sensor PCB and the AD7142.
If the application requires high resolution sensors, such as scroll
bars or wheels, software is required that runs on the host
processor. (No software is required for button sensors.) The
memory requirements for the host depend on the sensor, and
are typically 10 kB of code and 600 bytes of data memory.
Analog Devices supplies the sensor PCB footprint design
libraries to the customer based on the customer’s specifications,
and supplies any necessary software on an open-source basis.
OPERATING MODES
The AD7142 has three operating modes. Full power mode,
where the device is always fully powered, is suited for applications
where power is not a concern (for example, game consoles that
have an ac power supply). Low power mode, where the part
automatically powers down, is tailored to give significant power
savings over full power mode, and is suited for mobile applications
where power must be conserved. In shutdown mode, the part
shuts down completely.
The POWER_MODE bits (Bit 0 and Bit 1) of the control
register set the operating mode on the AD7142. The control
register is at Address 0x000. Table 8 shows the POWER_MODE
settings for each operating mode. To put the AD7142 into
shutdown mode, set the POWER_MODE bits to either 01 or 11.
Table 8. POWER_MODE Settings
POWER_MODE Bits
00
01
10
11
The power-on default setting of the POWER_MODE bits is 00,
full power mode.
Full Power Mode
In full power mode, all sections of the AD7142 remain fully
powered at all times. When a sensor is being touched, the
AD7142 processes the sensor data. If no sensor is touched, the
AD7142 measures the ambient capacitance level and uses this
data for the on-chip compensation routines. In full power
mode, the AD7142 converts at a constant rate. See the CDC
Conversion Sequence Time section for more information.
SENSOR PCB
Figure 19. Three Part Capacitance Sensing Solution
AD7142
Operating Mode
Full power mode
Full shutdown mode
Low power mode
Full shutdown mode
SPI OR I
2
C
HOST PROCESSOR
600 BYTES RAM
10kB ROM
1 MIPS
Rev. A | Page 13 of 72
Low Power Mode
When in low power mode, the AD7142 POWER_MODE bits
are set to 10 upon device initialization. If the external sensors
are not touched, the AD7142 reduces its conversion frequency,
thereby greatly reducing its power consumption. The part
remains in a reduced power state when the sensors are not
touched. Every LP_CONV_DELAY ms (200, 400, 600 or 800 ms),
the AD7142 performs a conversion and uses this data to update
the compensation logic. When an external sensor is touched,
the AD7142 begins a conversion sequence every 36 ms to read
back data from the sensors. In low power mode, the total
current consumption of the AD7142 is an average of the current
used during a conversion, and the current used when the
AD7142 is waiting for the next conversion to begin. For
example, when LP_CONV_DELAY l is 400 ms, the AD7142
typically uses 0.9 mA current for 36 ms, and 15 μA for 400 ms
of the conversion interval. Note that these conversion timings
can be altered through the register settings. See the CDC
Conversion Sequence Time section for more information.
The time taken for the AD7142 to go from a full power state to
a reduced power state, once the user stops touching the external
sensors, is configurable. The PWR_DWN_TIMEOUT bits, in
Ambient Compensation Ctrl 0 Register, at Address 0x002,
control the length of time the AD7142 takes before going into
the reduced power state, once the sensors are not touched.
EVERY LP_CONV_DELAY ms
CONVERSION SEQUENCE
UPDATE COMPENSATION
LOGIC DATA PATH
Figure 20. Low Power Mode Operation
NO
AND INITIALIZATION
POWER_MODE = 10
AD7142 SETUP
TOUCHED?
SENSOR
ANY
YES
YES
CONVERSION SEQUENCE
SENSOR READBACK
EVERY 36ms FOR
PROXIMITY TIMER
ANY SENSOR
COUNT DOWN
TOUCHED?
NO
AD7142
TIMEOUT