EVAL-AD7142EB View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

EVAL-AD7142EB

Programmable Capacitance-to-Digital Converter with Environmental Compensation

Analog Devices 

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

register bits control the length of the calibration disable period

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

AD7142

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)

Register

Length Register Address

FP_PROXIMITY_CNT

4 bits 0x002

LP_PROXIMITY_CNT

4 bits 0x002

FP_PROXIMITY_RECAL

8 bits 0x004

LP_PROXIMITY_RECAL

6 bits 0x004

PROXIMITY_RECAL_LVL

8 bits 0x003

PROXIMITY_DETECTION_RATE 6 bits 0x003

Description

Full power mode proximity control

Low power mode proximity control

Full power mode proximity recalibration control

Low power mode proximity recalibration control

Proximity recalibration level

Proximity detection rate

CDC CONVERSION SEQUENCE

(INTERNAL)

PROXIMITY DETECTION

(INTERNAL)

USER APPROACHES

SENSOR HERE

USER LEAVES SENSOR

AREA HERE

1 2 3 4 5 6 7 8 9 10 11 1213 14 15 16

tCALDIS

tCONV_FP

CALIBRATION

(INTERNAL)

CALIBRATION DISABLED

CALIBRATION ENABLED

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

Rev. 0 | Page 17 of 68

Share Link: