AD7147A Analog Devices, AD7147A Datasheet - Page 18

• 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
NONCONTACT PROXIMITY DETECTION
The AD7147A internal signal processing continuously monitors
all capacitance sensors for noncontact proximity detection. This
feature provides the ability to detect when a user is approaching
a sensor, at which time all internal calibration is immediately
disabled while the AD7147 is automatically configured to detect
a valid contact.
The proximity control register bits are described in Table 13. The
FP_PROXIMITY_CNT and LP_PROXIMITY_CNT register
bits control the length of the calibration disable period after the
user stops touching the sensor and is not in close proximity to
the sensor during full or low power mode. The calibration is
disabled during this period and then enabled again. Figure 29
and Figure 30 show examples of how these register bits are
used to set the calibration disable periods for the full and low
power modes.
The calibration disable period in full power mode is the value
of the FP_PROXIMITY_CNT multiplied by 16 multiplied by
the time for one conversion sequence in full power mode. The
calibration disable period in low power mode is the value of the
LP_PROXIMITY_CNT multiplied by 4 multiplied by the time
for one conversion sequence in low power mode.
RECALIBRATION
In certain situations, for example, when a user hovers over a
sensor for a long time, the proximity flag can be set for a long
period. The environmental calibration on the AD7147A is sus-
pended while proximity is detected, but changes may occur
to the ambient capacitance level during the proximity event.
This means that the ambient value stored on the AD7147A no
longer represents the actual ambient value. In this case, even
when the user is not in close proximity to the sensor, the prox-
imity flag may still be set. This situation can occur if the user
interaction creates some moisture on the sensor, causing the
new sensor ambient value to be different from the expected
value. In this situation, the AD7147A automatically forces a
recalibration internally. This ensures that the ambient values
are recalibrated, regardless of how long the user hovers over
the sensor. A recalibration ensures maximum AD7147A sensor
performance.
Rev. B | Page 18 of 68
The AD7147A recalibrates automatically when the measured CDC
value exceeds the stored ambient value by an amount determined
by the PROXIMITY_RECAL_LVL bits for a set period of time
known as the recalibration timeout. In full power mode, the recali-
bration timeout is controlled by FP_PROXIMITY_RECAL; in
low power mode, by LP_PROXMTY_RECAL.
The recalibration timeout in full power mode is the value of
FP_PROXIMITY_RECAL multiplied by the time for one
conversion sequence in full power mode. The recalibration time-
out in low power mode is the value of LP_PROXIMITY_RECAL
multiplied by the time for one conversion sequence in low
power mode.
Figure 31 and Figure 32 show examples of how the FP_
PROXIMITY_RECAL and LP_PROXIMITY_RECAL register
bits control the timeout period before a recalibration while
operating in the full and low power modes. In these examples,
a user approaches a sensor and then leaves, but the proximity
detection remains active. The measured CDC value exceeds the
stored ambient value by the amount set in the PROXIMITY_
RECAL_LVL bits for the entire timeout period. The sensor is
automatically recalibrated at the end of the timeout period.
PROXIMITY SENSITIVITY
The fast filter in Figure 33 is used to detect when someone is
close to the sensor (proximity). Two conditions, detected by
Comparator 1 and Comparator 2, set the internal proximity
detection signal: Comparator 1 detects when a user is approach-
ing or leaving a sensor, and Comparator 2 detects when a user
hovers over a sensor or approaches a sensor very slowly.
The sensitivity of Comparator 1 is controlled by the PROXIMITY_
DETECTION_RATE bits (Address 0x003). For example, if
PROXIMITY_DETECTION_RATE is set to 4, the Proximity 1
signal is set when the absolute difference between WORD1 and
WORD3 exceeds (4 × 16) LSB codes.
The PROXIMITY_RECAL_LVL bits (Address 0x003) control
the sensitivity of Comparator 2. For example, if PROXIMITY_
RECAL_LVL is set to 75, the Proximity 2 signal is set when the
absolute difference between the fast filter average value and the
ambient value exceeds (75 × 16) LSB codes.