AD7142 datasheet(17/68 Pages) AD

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AD7142

Rev. 0 | Page 17 of 68

NONCONTACT PROXIMITY DETECTION

The AD7142 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 AD7142 is automatically configured to detect

a valid contact.

The proximity control register bits are described in Table 12. The

FP_PROXIMITY_CNT register bits and LP_PROXIMITY_CNT

after proximity is detected. The calibration is disabled during this

time and enabled again at the end of this period provided that the

user is no longer approaching, or in contact with, the sensor.

Figure 26 and Figure 27 show examples of how these registers are

used to set the full and low power mode calibration disable

periods.

RECALIBRATION

In the event of a very long proximity detection event, such as a

user hovering over a sensor for a long period of time, the

FP_PROXIMITY_RECAL Bits[9:0] and LP_PROXIMITY_RECAL

Bits[15:10] in Register 0x004 can be applied to force a recalibration.

This ensures that the ambient values are recalibrated regardless of how

long the user hovers over a sensor. A recalibration ensures maximum

AD7142 sensor performance. Figure 28 and Figure 29 show examples

of using the FP_PROXIMITY_RECAL and

LP_PROXIMITY_RECAL register bits to force a recalibration while

operating in the full and low power modes. These figures show a user

approaching a sensor followed by the user leaving the sensor while the

proximity detection remains active after the user leaves the sensor. This

situation could occur if the user interaction creates some moisture on

the sensor causing the new sensor value to be different from the

expected value. In this case, the internal recalibration is applied to

automatically recalibrate the sensor. The force recalibration event takes

two interrupt cycles, therefore it should not be set again during this

interval.

PROXIMITY SENSITIVITY

Figure 30 describes the two conditions that set the internal

proximity detection signal using Comparator 1 and

Comparator 2. Comparator 1 detects when a user is approach-

ing a sensor. The PROXIMITY_DETECTION_RATE register

controls the sensitivity of Comparator 1. 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 four LSB codes. Comparator 2 detects when a

user hovers over a sensor or approaches a sensor very slowly.

The PROXIMITY_RECAL_LVL register (Address 0x003)

controls 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 LSB codes.

Table 12. Proximity Control Registers (See Figure 30)

Length

Description

FP_PROXIMITY_CNT

4 bits

0x002

Full power mode proximity control

LP_PROXIMITY_CNT

4 bits

0x002

Low power mode proximity control

FP_PROXIMITY_RECAL

8 bits

0x004

Full power mode proximity recalibration control

LP_PROXIMITY_RECAL

6 bits

0x004

Low power mode proximity recalibration control

PROXIMITY_RECAL_LVL

8 bits

0x003

Proximity recalibration level

PROXIMITY_DETECTION_RATE

6 bits

0x003

Proximity detection rate

CALIBRATION ENABLED

CALIBRATION DISABLED

PROXIMITY DETECTION

(INTERNAL)

CALIBRATION

(INTERNAL)

tCALDIS

tCONV_FP

1 2 34 5 6 78 9 10 11 1213 14 15 16

CDC CONVERSION SEQUENCE

(INTERNAL)

USER LEAVES SENSOR

AREA HERE

USER APPROACHES

SENSOR HERE

Figure 26. Full Power Mode Proximity Detection Example with FP_PROXIMITY = 1

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