AD7148 Analog Devices, AD7148 Datasheet - Page 17

• 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

NONCONTACT PROXIMITY DETECTION
The AD7148 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 AD7148 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
(Address 0x002[11:4]) control the length of the calibration disable
period after the user leaves the sensor and proximity is no longer
active in full and low power modes.
Table 12. Proximity Control Registers (See Figure 30)
Bits
FP_PROXIMITY_CNT
LP_PROXIMITY_CNT
FP_PROXIMITY_RECAL
LP_PROXIMITY_RECAL
PROXIMITY_RECAL_LVL
PROXIMITY_DETECTION_RATE
CDC CONVERSION SEQUENCE
CDC CONVERSION SEQUENCE
PROXIMITY DETECTION
PROXIMITY DETECTION
CALIBRATION
CALIBRATION
(INTERNAL)
(INTERNAL)
(INTERNAL)
(INTERNAL)
(INTERNAL)
(INTERNAL)
NOTES
1. SEQUENCE CONVERSION TIME
2. PROXIMITY IS SET WHEN USER APPROACHES THE SENSOR AT WHICH TIME THE INTERNAL CALIBRATION IS DISABLED.
3.
Length
4 bits
4 bits
10 bits
6 bits
8 bits
6 bits
Figure 26. Full Power Mode Proximity Detection Example with FP_PROXIMITY_CNT = 1
t
CALDIS
USER APPROACHES
USER APPROACHES
Figure 27. Low Power Mode Proximity Detection with LP_PROXIMITY_CNT = 4
SENSOR HERE
SENSOR HERE
= (
t
CONV_LP
Register Address
0x002[7:4]
0x002[11:8]
0x004[9:0]
0x004[15:10]
0x003[7:0]
0x003[13:8]
1 2 3 4 5 6 7 8 9 10 11 1213 14 15 16
1 2 3 4 5
× LP_PROXIMITY_CNT × 4)
SENSOR AREA HERE
USER LEAVES
USER LEAVES SENSOR
6 7 8 9 10 11 12 13 14 15 16
t
CONV_LP
CALIBRATION DISABLED
AREA HERE
Rev. A | Page 17 of 56
CALIBRATION DISABLED
=
t
CONV_FP
Description
Calibration disable time in full power mode.
Calibration disable time in low power mode.
Full power mode proximity recalibration time.
Low power mode proximity recalibration time.
Proximity recalibration level. This value, multiplied by 16, controls the
sensitivity of Comparator 2 in Figure 30.
Proximity detection rate. This value, multiplied by 16, controls the
sensitivity of Comparator 1 in Figure 30.
+ LP_CONV_DELAY
The calibration is disabled during this time and is 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
show examples of how these registers are used to set the full
and low power mode calibration disable periods.
The calibration disable period in full power mode is equal to
FP_PROXIMITY_CNT × 16 × time taken for one conversion
sequence in full power mode.
The calibration disable period in low power mode is equal to
LP_PROXIMITY_CNT × 4 × time taken for one conversion
sequence in low power mode.
t
CALDIS
t
CALDIS
1718 19 20 21 22 23 24
CALIBRATION ENABLED
CALIBRATION ENABLED
t
CONV_LP
t
CONV_FP
AD7148