AD7147 datasheet(25/73 Pages) AD

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AD7147

Data Sheet

Rev. D | Page 24 of 72

ENVIRONMENTAL CALIBRATION

STAGEx_LOW_THRESHOLD

STAGEx_HIGH_THRESHOLD

CDC AMBIENT VALUE

CHANGING ENVIRONMENTAL CONDITIONS

SENSOR 1 INT

ASSERTED

SENSOR 2 INT

ASSERTED

The AD7147 provides on-chip capacitance sensor calibration to

automatically adjust for environmental conditions that have an

effect on the ambient levels of the capacitance sensor. The output

levels of the capacitance sensor are sensitive to temperature,

humidity, and, in some cases, dirt.

The AD7147 achieves optimal and reliable sensor performance

by continuously monitoring the CDC ambient levels and compen-

sating for any environmental changes by adjusting the values of

the STAGEx_HIGH_THRESHOLD register and the STAGEx_

LOW_THRESHOLD registers as described in the Threshold

Equations section. The CDC ambient level is defined as the

output level of the capacitance sensor during periods when the

user is not approaching or in contact with the sensor.

Figure 35. Ideal Sensor Behavior with a Constant Ambient Level

CAPACITANCE SENSOR BEHAVIOR WITHOUT

CALIBRATION

After the AD7147 is configured, the compensation logic runs

automatically with each conversion when the AD7147 is not

being touched. This allows the AD7147 to compensate for

rapidly changing environmental conditions.

Figure 36 shows the typical behavior of a capacitance sensor

when calibration is not applied. This figure shows ambient

levels drifting over time as environmental conditions change. As

a result of the initial threshold levels remaining constant while

the ambient levels drift upward, Sensor 2 fails to detect a user

contact in this example.

The ambient compensation control registers provide the host

with 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 for each stage.

The Capacitance Sensor Behavior with Calibration section

describes how the AD7147 adaptive calibration algorithm

prevents such errors from occurring.

Figure 35 shows an example of the ideal behavior of a capaci-

tance sensor, 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

and are based on the threshold sensitivity settings and the

ambient value. These values are sufficient to detect a sensor

contact and result in the AD7147 asserting the INT output

when the threshold levels are exceeded.

CDC AMBIENT

VALUE DRIFTING

CHANGING ENVIRONMENTAL CONDITIONS

STAGEx_HIGH_THRESHOLD

STAGEx_LOW_THRESHOLD

SENSOR 1 INT

ASSERTED

SENSOR 2 INT

NOT ASSERTED

Figure 36. Typical Sensor Behavior Without Calibration

AD7147 Datasheet(PDF) 25 Page - Analog Devices

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