AD7142 Analog Devices, AD7142 Datasheet - Page 22

• 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
ENVIRONMENTAL CALIBRATION
The AD7142 provides on-chip capacitance sensor calibration to
automatically adjust for environmental conditions that have an
effect on the capacitance sensor ambient levels. Capacitance
sensor output levels are sensitive to temperature, humidity, and
in some cases, dirt. The AD7142 achieves optimal and reliable
sensor performance by continuously monitoring the CDC
ambient levels and correcting for any changes by adjusting the
STAGE_HIGH_THRESHOLD and STAGE_LOW_ THRESHOLD
register values, as described in Equation 1 and Equation 2. The
CDC ambient level is defined as the capacitance sensor output
level during periods when the user is not approaching or in
contact with the sensor.
The compensation logic runs automatically on every conversion
after configuration when the AD7142 is not being touched. This
allows the AD7142 to account for rapidly changing environ-
mental conditions.
The ambient compensation control registers give the host access
to general setup and controls for the compensation algorithm.
The RAM stores the compensation data for each conversion
stage, as well as setup information specific to each stage.
Figure 33 shows an example of an ideal capacitance sensor
behavior where the CDC ambient level remains constant
regardless of the environmental conditions. The CDC output
shown is for a pair of differential button sensors, where one
sensor caused an increase, and the other a decrease in measured
capacitance when activated. The positive and negative sensor
threshold levels are calculated as a percentage of the
STAGE_OFFSET_HIGH and STAGE_OFFSET_LOW values
based on the threshold sensitivity settings and the ambient
value. These values are sufficient to detect a sensor contact,
resulting with the AD7142 asserting the INT output when the
threshold levels are exceeded.
Figure 33. Ideal Sensor Behavior with a Constant Ambient Level
CHANGING ENVIRONMENTAL CONDITIONS
SENSOR 2 INT
ASSERTED
SENSOR 1 INT
ASSERTED
t
STAGE_HIGH_THRESHOLD
CDC AMBIENT VALUE
STAGE_LOW_THRESHOLD
Rev. A | Page 22 of 72
CAPACITANCE SENSOR BEHAVIOR WITHOUT
CALIBRATION
Figure 34 shows the typical behavior of a capacitance sensor
with no applied calibration. This figure shows ambient levels
drifting over time as environmental conditions change. The
ambient level drift has resulted in the detection of a missed user
contact on Sensor 2. This is a result of the initial low offset level
remaining constant when the ambient levels drifted upward
beyond the detection range.
The Capacitance Sensor Behavior with Calibration section
describes how the AD7142 adaptive calibration algorithm
prevents errors such as this from occurring.
CAPACITANCE SENSOR BEHAVIOR WITH
CALIBRATION
The AD7142 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 described 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 AD7142 under dynamic environmental
conditions. Figure 35 shows a simplified example of how the
AD7142 applies the adaptive calibration process resulting in no
interrupt errors under changing CDC ambient levels due to
environmental conditions.
CHANGING ENVIRONMENTAL CONDITIONS
Figure 34. Typical Sensor Behavior Without Calibration Applied
NOT ASSERTED
SENSOR 2 INT
SENSOR 1 INT
ASSERTED
t
STAGE_HIGH_THRESHOLD
CDC AMBIENT
VALUE DRIFTING
STAGE_LOW_THRESHOLD