AD7148 Analog Devices, AD7148 Datasheet - Page 18

• Manufacturer Part Number: AD7148
• Manufacturer: Analog Devices

Specifications of AD7148

• Resolution (bits): 16bit
• # Chan: 8
• Sample Rate: 250kSPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: SE-Uni
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7148
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ambient value stored on the AD7148 no longer represents the actual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.
CDC CONVERSION SEQUENCE
RECALIBRATION COUNTER
PROXIMITY DETECTION
Figure 28. Full Power Mode Proximity Detection with Forced Recalibration Example with FP_PROXIMITY_CNT = 1 and FP_PROXIMITY_RECAL = 40
CALIBRATION
(INTERNAL)
(INTERNAL)
(INTERNAL)
(INTERNAL)
NOTES
1. SEQUENCE CONVERSION TIME
2.
3.
4.
t
t
t
CALDIS
RECAL_TIMEOUT
RECAL
USER APPROACHES
SENSOR HERE
= 2 ×
=
t
CONV_FP
t
CONV_FP
=
t
CONV_FP
× FP_PROXIMITY_CNT × 16.
USER LEAVES SENSOR
CALIBRATION DISABLED
.
AREA HERE
× FP_PROXIMITY_RECAL.
t
CONV_FP
16
t
CALDIS
DETERMINED FROM TABLE 9.
Rev. A | Page 18 of 56
30
MEASURED CDC VALUE > STORED AMBIENT
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.
BY PROXIMITY_RECAL _LVL
RECALIBRATION TIME-OUT
t
RECAL_TIMEOUT
t
RECAL
70
CALIBRATION ENABLED
t
CONV_FP