AD7142_17 datasheet(25/70 Pages) AD

No de pièce	AD7142

Description	Programmable Controller for Capacitance Touch Sensors

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Data Sheet

AD7142

Rev. B | Page 25 of 70

ADAPTIVE THRESHOLD AND SENSITIVITY

The AD7142 provides an on-chip self-learning adaptive

threshold and sensitivity algorithm. This algorithm continu-

ously monitors the output levels of each sensor and automatically

rescales the threshold levels proportionally to the sensor area

covered by the user. As a result, the AD7142 maintains optimal

threshold and sensitivity levels for all types of users regardless

of their finger sizes.

The threshold level is always referenced from the ambient level

and is defined as the CDC converter output level that must be

exceeded for a valid sensor contact. The sensitivity level is

defined as how sensitive the sensor is before a valid contact

is registered.

Figure 36 provides an example of how the adaptive threshold and

sensitivity algorithm works. 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. On

configuration, initial estimates are supplied for both

STAGE_OFFSET_HIGH and STAGE_OFFSET_LOW after

which the calibration engine automatically adjusts the

STAGE_HIGH_THRESHOLD and STAGE_LOW_

THRESHOLD values for sensor response.

The AD7142 tracks the average maximum and minimum values

measured from each sensor. These values give an indication of

how the user is interacting with the sensor. A large finger gives

a large average maximum or minimum value, and a small finger

gives smaller values. When the average maximum or minimum

value changes, the threshold levels are rescaled to ensure that

the threshold levels are appropriate for the current user. Figure 37

shows how the minimum and maximum sensor responses are

tracked by the on-chip logic.

Reference A in Figure 36 shows an undersensitive threshold

level for a small finger user, demonstrating the disadvantages of

a fixed threshold level.

By enabling the adaptive threshold and sensitivity algorithm,

the positive and negative threshold levels are determined by the

POS_THRESHOLD_SENSITIVITY and NEG_THRESHOLD_

SENSITIVITY register values and the most recent average

maximum sensor output value. These registers can be used to

select 16 different positive and negative sensitivity levels

ranging between 25% and 95.32% of the most recent average

maximum output level referenced from the ambient value. The

smaller the sensitivity percentage setting, the easier it is to

trigger a sensor activation. Reference B shows that the positive

adaptive threshold level is set at almost mid-sensitivity with a

62.51% threshold level by setting POS_THRESHOLD_

SENSITIVITY = 1000. Figure 36 also provides a similar

example for the negative threshold level with NEG_

THRESHOLD_SENSITIVITY = 0001.

AMBIENT LEVEL

AVERAGE MAX VALUE

STAGE_OFFSET_HIGH

25%

95.32%

62.51% = POS

ADAPTIVE

THRESHOLD LEVEL

25%

62.51% = POS ADAPTIVE

THRESHOLD LEVEL

95.32%

NEG ADAPTIVE THRESHOLD LEVEL = 39.08%

25%

95.32%

25%

95.32%

SENSOR CONTACTED

BY SMALL FINGER

AVERAGE MAX VALUE

STAGE_OFFSET_LOW

NEG ADAPTIVE THRESHOLD LEVEL = 39.08%

SENSOR CONTACTED

BY LARGE FINGER

STAGE_OFFSET_HIGH

IS UPDATED

STAGE_OFFSET_HIGH

IS UPDATED HERE

STAGE_OFFSET_LOW

IS UPDATED HERE

STAGE_OFFSET_LOW

IS UPDATED HERE

A

B

Figure 36. Threshold Sensitivity Example with POS_THRESHOLD_SENSITIVITY = 1000 and NEG_THRESHOLD_SENSITIVITY = 0011

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