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AD7148

Rev. 0 | Page 22 of 56

ENVIRONMENTAL CALIBRATION

The AD7148 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 AD7148 achieves optimal and reliable

sensor performance by continuously monitoring the CDC ambient

levels and correcting for any changes by adjusting the STAGEx_

HIGH_THRESHOLD and STAGEx_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 AD7148 is not being touched, which

allows the AD7148 to account for rapidly changing environmental

conditions.

The ambient compensation control registers give the host access

to general setup and controls for the compensation algorithm.

On-chip 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 caused a decrease in

measured capacitance when activated. The positive and negative

sensor threshold levels are calculated as a percentage of the

STAGEx_OFFSET_HIGH and STAGEx_OFFSET_LOW values

based on the threshold sensitivity settings and the ambient value.

These values are sufficient to detect a sensor contact, resulting

in the AD7148 asserting the INT output when threshold levels

are exceeded.

STAGEx_LOW_THRESHOLD

STAGEx_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 results 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.

CDC AMBIENT

VALUE DRIFTING

SENSOR 1 INT

ASSERTED

SENSOR 2 INT

NOT ASSERTED

CHANGING ENVIRONMENTAL CONDITIONS

STAGEx_HIGH_THRESHOLD

STAGEx_LOW_THRESHOLD

Figure 32. Typical Sensor Behavior Without Calibration Applied

The Capacitance Sensor Behavior with Calibration section

describes how the AD7148 adaptive calibration algorithm

prevents errors such as this from occurring.

CAPACITANCE SENSOR BEHAVIOR WITH

CALIBRATION

The AD7148 on-chip adaptive calibration algorithm prevents

sensor detection errors such as the one shown in Figure 32.

Error prevention is accomplished by monitoring CDC ambient

levels and readjusting the initial STAGEx_OFFSET_HIGH and

STAGEx_OFFSET_LOW values according to the amount of

ambient drift measured on each sensor. The internal STAGEx_

HIGH_THRESHOLD and STAGEx_LOW_THRESHOLD

values described in Equation 1 and Equation 2 are automatically

updated based on the new values of STAGEx_OFFSET_HIGH

and STAGEx_OFFSET_LOW. This closed-loop routine ensures

the reliability and repeatable operation of every sensor

connected to the AD7148 under dynamic environmental

conditions. Figure 33 shows a simplified example of how the

AD7148 applies the adaptive calibration process resulting in no

interrupt errors under changing CDC ambient levels due to

environmental conditions.

AD7148 Datasheet(PDF) 22 Page - Analog Devices

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