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AD7148

Rev. 0 | Page 18 of 56

RECALIBRATION

In certain situations, the proximity flag can be set for a long period: for

example, when a user hovers over a sensor for a long time. The

environmental calibration on the AD7148 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ambient value stored on the AD7148 no longer represents the actual

ambient value. In this case, even when the user has left the sensor, the

proximity flag may still be set. This situation could occur if 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 AD7148 automatically forces an internal recalibration,

ensuring that the ambient values are recalibrated, regardless of how

long the user hovers over a sensor. The recalibration ensures

maximum sensor performance.

The AD7148 recalibrates automatically when the measured CDC

value exceeds the stored ambient value by an amount determined by

the PROXIMITY_RECAL_LVL bits (Address 0x003[7:0]) 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, the timeout is

controlled by LP_PROXIMTY_RECAL.

The recalibration timeout in full power mode is the value of the

FP_PROXIMITY_RECAL multiplied by the time taken for one

conversion sequence in full power mode.

The recalibration timeout in low power mode is the value of the

LP_PROXIMITY_RECAL multiplied by the time taken for one

conversion sequence in low power mode.

Figure 28 and Figure 29 show examples of how the FP_

PROXIMITY_RECAL and LP_PROXIMITY_RECAL register

bits (Address 0x004[15:0]) control the timeout period before a

recalibration while operating in the full power 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. 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.

CALIBRATION ENABLED

tRECAL_TIMEOUT

tCONV_FP

MEASURED CDC VALUE > STORED AMBIENT

BY PROXIMITY_RECAL _LVL

RECALIBRATION TIME-OUT

PROXIMITY DETECTION

(INTERNAL)

CALIBRATION

(INTERNAL)

CDC CONVERSION SEQUENCE

(INTERNAL)

RECALIBRATION COUNTER

(INTERNAL)

NOTES

1. SEQUENCE CONVERSION TIME

tCONV_FP DETERMINED FROM TABLE 9.

tCALDIS = tCONV_FP × FP_PROXIMITY_CNT × 16.

tRECAL_TIMEOUT = tCONV_FP × FP_PROXIMITY_RECAL.

tRECAL = 2 × tCONV_FP.

USER APPROACHES

SENSOR HERE

USER LEAVES SENSOR

AREA HERE

tCALDIS

tRECAL

CALIBRATION DISABLED

Figure 28. Full Power Mode Proximity Detection with Forced Recalibration Example with FP_PROXIMITY_CNT = 1 and FP_PROXIMITY_RECAL = 40

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