AD7142 Analog Devices, AD7142 Datasheet - Page 18

• Manufacturer Part Number: AD7142
• Description: Programmable Controller for Capacitance Touch Sensors
• Manufacturer: Analog Devices

Specifications of AD7142

• Resolution (bits): 16bit
• # Chan: 14
• Sample Rate: 250kSPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: ± 2 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7142
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 and 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 the user leaves the sensor and
proximity is no longer active, in full and low power modes. 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 28 and
Figure 29 show examples of how these registers are used to set
the full and low power mode calibration disable periods.
Calibration disable period in full power mode =
FP_PROXIMITY_CNT × 16 × Time taken for one conversion
sequence in full power mode
Calibration disable period in low power mode =
LP_PROXIMITY_CNT × 4 × Time taken for one conversion
sequence in low power mode
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 AD7142 is
suspended when proximity is detected, but changes may occur
to the ambient capacitance level during the proximity event.
This means the ambient value stored on the AD7142 no longer
represents the actual ambient value.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 the 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
AD7142 automatically forces a recalibration internally. 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.
The AD7142 recalibrates automatically when the measured
CDC value exceeds the stored ambient value by an amount
Table 12. Proximity Control Registers (See Figure 32)
Register
FP_PROXIMITY_CNT
LP_PROXIMITY_CNT
FP_PROXIMITY_RECAL
LP_PROXIMITY_RECAL
PROXIMITY_RECAL_LVL
PROXIMITY_DETECTION_RATE
Length
4 bits
4 bits
8 bits
6 bits
8 bits
6 bits
Register Address
0x002 [7:4]
0x002 [11:8]
0x004 [9:0]
0x004 [15:10]
0x003 [13:8]
0x003 [7:0]
Rev. A | Page 18 of 72
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
Proximity detection rate
determined by PROXIMITY_RECAL_LVL, for a set period of
time known as the recalibration timeout. In full power mode, the
recalibration timeout is controlled by FP_PROXIMITY_RECAL,
and in low power mode, by LP_PROXIMITY_RECAL.
Recalibration timeout in full power mode =
FP_PROXIMITY_RECAL × Time taken for one conversion
sequence in full power mode
Recalibration timeout in low power mode =
LP_PROXIMITY_RECAL × Time taken for one conversion
sequence in low power mode
Figure 30 and Figure 31 show examples of how the
FP_PROXIMITY_RECAL and LP_PROXIMITY_RECAL
register bits control the timeout period before a recalibration,
operating in the full and low power modes. These figures show a
user approaching a sensor followed by the user leaving the sensor
and 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. The forced recalibration takes
two interrupt cycles, therefore, it should not be set again during
this interval.
PROXIMITY SENSITIVITY
The fast filter in Figure 32 is used to detect when someone is
close to the sensor (proximity). Two conditions set the internal
proximity detection signal using Comparator 1 and
Comparator 2. Comparator 1 detects when a user is
approaching 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.