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AD7147
(Rev.:Rev0)

CapTouch™ Programmable Controller for Single-Electrode Capacitance Sensors

Analog Devices 

AD7147

NONCONTACT PROXIMITY DETECTION

The AD7147 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 12. 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 registers 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 AD7147 is

suspended 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 AD7147 no

longer represents the actual ambient value. In this case, even

when the user is not in close proximity to the sensor, the

proximity 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 AD7147 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 AD7147 sensor

performance.

The AD7147 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

recalibration 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 the

FP_PROXIMITY_RECAL multiplied by the time for one con-

version sequence in full power mode. The recalibration timeout in

low power mode is the value of the 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

approaching 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. 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 sensitivity of Comparator 2 is controlled by the

PROXIMITY_RECAL_LVL bits (Address 0x003). 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.

Rev. 0 | Page 18 of 68

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