EVAL-AD7142-1EBZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7142-1EBZ' PDF : 73 Pages View PDF
AD7142
Complete Solution for Capacitance Sensing
Analog Devices 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.
SENSOR PCB
AD7142
SPI OR I2C
HOST PROCESSOR
1 MIPS
10kB ROM
600 BYTES RAM
Figure 17. Three Part Capacitance Sensing Solution
Analog Devices supplies the sensor PCB design to the customer
based on the customer’s specifications, and supplies any necessary
software on an open-source basis. Standard sensor designs are
also available as PCB library components.
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
Operating Mode
00
Full power mode
01
Full shutdown mode
10
Low power mode
11
Full shutdown mode
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. While 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.
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 low power state while the sensors are not touched.
Every 400 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 40 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 while the AD7142 is waiting for the next
conversion to begin. For example, when the low power mode
conversion interval is 400 ms, the AD7142 typically uses 0.9 mA
current for 40 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.)
AD7142 SETUP
AND INITIALIZATION
POWER_MODE = 10
NO
ANY
SENSOR
YES
TOUCHED?
CONVERSION SEQUENCE
EVERY LP_CONV_DELAY ms
UPDATE COMPENSATION
LOGIC DATA PATH
CONVERSION SEQUENCE
EVERY 36ms FOR
SENSOR READBACK
YES ANY SENSOR
TOUCHED?
NO
PROXIMITY TIMER TIMEOUT
COUNT DOWN
Figure 18. Low Power Mode Operation
Rev. 0 | Page 12 of 68
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