AD7143 Analog Devices, AD7143 Datasheet - Page 23

• Manufacturer Part Number: AD7143
• Description: Programmable Controller for Capacitance Touch Sensors
• Manufacturer: Analog Devices

Specifications of AD7143

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

CAPACITANCE SENSOR BEHAVIOR WITH
CALIBRATION
The AD7143 on-chip adaptive calibration algorithm prevents
sensor detection errors, such as the one shown in Figure 34.
This is achieved by monitoring the CDC ambient levels and
readjusting the initial STAGE_OFFSET_HIGH and
STAGE_OFFSET_LOW values according to the amount of
ambient drift measured on each sensor.
The internal STAGE_HIGH_THRESHOLD and
STAGE_LOW_THRESHOLD values, shown in Equation 1 and
Equation 2, are automatically updated based on the new
STAGE_OFFSET_HIGH and STAGE_OFFSET_LOW values.
This closed-loop routine ensures the reliability and repeatable
operation of every sensor connected to the AD7143 under
dynamic environmental conditions. Figure 35 shows a
simplified example of how the AD7143 applies the adaptive
calibration process resulting in no interrupt errors under
changing CDC ambient levels due to environmental conditions.
On-Chip Logic Stage High Threshold Calculation
On-Chip Logic Stage Low Threshold Calculation
⎛
⎜
⎜
⎜
⎜
⎝
⎛
⎜
⎜
⎜
⎜
⎝
STAGE
STAGE
⎛
⎜
⎝
⎛
⎜
⎝
STAGE
STAGE
_
_
HIGH
LOW
_
_
OFFSET
OFFSET
_
_
THRESHOLD
THRESHOLD
_
_
HIGH
LOW
16
−
16
−
=
=
STAGE
STAGE
STAGE
STAGE
_
_
_
OFFSET
OFFSET
_
SF
SF
4
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4
_
_
AMBIENT
AMBIENT
_
_
LOW
HIGH
⎞
⎟
⎠
+
⎞
⎟
⎠
+
⎞
⎟
⎟
⎟
⎟
⎠
⎛
⎜
⎝
⎞
⎟
⎟
⎟
⎟
⎠
⎛
⎜
⎝
×
STAGE
×
STAGE
NEG
Figure 35. Typical Sensor Behavior with Calibration Applied on the Data Path
POS
1
2
3
4
5
6
INITIAL STAGE_OFFSET_HIGH REGISTER VALUE.
POST CALIBRATED REGISTER STAGE_HIGH_THRESHOLD.
POST CALIBRATED REGISTER STAGE_HIGH_THRESHOLD.
INITIAL STAGE_LOW_THRESHOLD.
POST CALIBRATED REGISTER STAGE_LOW_THRESHOLD.
POST CALIBRATED REGISTER STAGE_LOW_THRESHOLD.
_
_
THRESHOLD
CHANGING ENVIRONMENTAL CONDITIONS
_
_
THRESHOLD
1
4
OFFSET
OFFSET
4
4
_
_
LOW
SENSOR 2 INT
HIGH
_
ASSERTED
_
SENSITIVIT
SENSITIVIT
⎞
⎟
⎠
⎞
⎟
⎠
+
SENSOR 1 INT
2
5
+
ASSERTED
Y
Y
(1)
(2)
3
6
t
STAGE_HIGH_THRESHOLD
(POST CALIBRATED
REGISTER VALUE)
CDC AMBIENT
VALUE DRIFTING
STAGE_LOW_THRESHOLD
(POST CALIBRATED
REGISTER VALUE)
AD7143