AD7142 datasheet(24/68 Pages) AD

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AD7142

Rev. 0 | Page 24 of 68

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 34 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.

Reference A in Figure 34 shows an under sensitive 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 34 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

Figure 34. Threshold Sensitivity Example with POS_THRESHOLD_SENSITIVITY = 1000 and NEG_THRESHOLD_SENSITIVITY = 0011

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