AD7147A Analog Devices, AD7147A Datasheet - Page 12

• Manufacturer Part Number: AD7147A
• Manufacturer: Analog Devices

Specifications of AD7147A

• Resolution (bits): 16bit
• # Chan: 13
• Sample Rate: 111SPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: Capacitive
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7147A
Registering a Sensor Activation
When a user approaches a sensor, the total capacitance associated
with that sensor changes and is measured by the AD7147A. If
the change causes a set threshold to be exceeded, the AD7147A
interprets this as a sensor activation.
On-chip threshold limits are used to determine when a sensor
activation occurs. Figure 20 shows the change in CDC_RESULT_Sx
when a user activates a sensor. The sensor is deemed to be active
only when the value of CDC_RESULT_Sx is either greater than the
value of STAGEx_HIGH_THRESHOLD or less than the value
of STAGEx_LOW_THRESHOLD.
In Figure 20, two sensor activations are shown. Sensor Active A
occurs when a sensor is connected to the positive input of the
converter. In this case, when a user activates the sensor, there is an
increase in CDC code, and the value of CDC_RESULT_Sx exceeds
that of STAGEx_HIGH_THRESHOLD. Sensor Active B occurs
when the sensor is connected to the negative input of the converter.
In this case, when a user activates the sensor, there is a decrease
in CDC code, and the value of CDC_RESULT_Sx becomes less
than the value of STAGEx_LOW_THRESHOLD.
For each conversion stage, the STAGEx_HIGH_THRESHOLD
and STAGEx_LOW_THRESHOLD registers are in Bank 3.
The values in these registers are updated automatically by the
AD7147A due to its environmental calibration and adaptive
threshold logic.
At power-up, the values in the STAGEx_HIGH_THRESHOLD
and STAGEx_LOW_THRESHOLD registers are the same as those
in the STAGEx_OFFSET_HIGH and STAGEx_OFFSET_LOW
registers in Bank 2. The user must program the STAGEx_OFFSET
_HIGH and STAGEx_OFFSET_LOW registers on device power-
up. See the Environmental Calibration section of the data sheet
for more information.
SENSOR ACTIVE A
Figure 20. Sensor Activation Thresholds
SENSOR ACTIVE B
CDC_RESULT_Sx
STAGEx_HIGH_THRESHOLD
AMBIENT OR
NO-TOUCH VALUE
STAGEx_LOW_THRESHOLD
Rev. B | Page 12 of 68
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 AD7147A.
If the application requires high resolution sensors such as scroll
bars or wheels, software that runs on the host processor is required.
The memory requirements for the host depend on the sensor
and are typically 10 kB of code and 600 bytes of data memory,
depending on the sensor type.
Analog Devices supplies the sensor PCB footprint design
libraries to the customer and supplies any necessary software on
an open-source basis.
BIAS PIN
This pin is connected internally to a bias node of the AD7147A.
To ensure correct operation of the AD7147A, connect a 100 nF
capacitor between the BIAS pin and ground. The voltage seen at
the BIAS pin is V
OPERATING MODES
The AD7147A 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 when no sensor is active, is tailored to provide
significant power savings compared with 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[1:0] of the power control register
(PWR_CONTROL, Address 0x000) set the operating mode on
the AD7147A. Table 8 shows the POWER_MODE settings for
each operating mode. To put the AD7147A 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.
SENSOR PCB
Figure 21. Three-Part Capacitance Sensing Solution
CC
/2.
AD7147A
Operating Mode
Full power mode
Shutdown mode
Low power mode
Shutdown mode
SPI OR I
2
C
HOST PROCESSOR
600 BYTES RAM
10kB ROM
1 MIPS