AD7142 datasheet(22/68 Pages) AD

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AD7142

Rev. 0 | Page 22 of 68

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. 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 31 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.

STAGE_LOW_THRESHOLD

STAGE_HIGH_THRESHOLD

CDC AMBIENT VALUE

CHANGING ENVIRONMENTAL CONDITIONS

SENSOR 1 INT

ASSERTED

SENSOR 2 INT

ASSERTED

Figure 31. Ideal Sensor Behavior with a Constant Ambient Level

CAPACITANCE SENSOR BEHAVIOR WITHOUT

CALIBRATION

Figure 32 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 while 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.

CCDC AMBIENT

VALUE DRIFTING

SENSOR 1 INT

ASSERTED

SENSOR 2 INT

NOT ASSERTED

CHANGING ENVIRONMENTAL CONDITIONS

STAGE_HIGH_THRESHOLD

STAGE_LOW_THRESHOLD

Figure 32. Typical Sensor Behavior Without Calibration Applied

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