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AD7148

Rev. 0 | Page 24 of 56

ADAPTIVE THRESHOLD AND SENSITIVITY

The AD7148 provides an on-chip self-learning adaptive threshold

and sensitivity algorithm. This algorithm continuously 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 AD7148 maintains optimal threshold and

sensitivity levels for all types of users, regardless of finger size.

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 STAGEx_

OFFSET_HIGH and STAGEx_OFFSET_LOW values, based

on the threshold sensitivity settings and the ambient value. On

configuration, initial estimates are supplied for both STAGEx_

OFFSET_HIGH and STAGEx_OFFSET_LOW, after which the

calibration engine automatically adjusts the STAGEx_HIGH_

THRESHOLD and STAGEx_LOW_THRESHOLD values for

sensor response.

The AD7148 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 35

shows how the minimum and maximum sensor responses are

tracked by the on-chip logic.

Reference A in Figure 34 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 values and the most recent average maximum

sensor output value. These bits 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 = 0011.

AMBIENT LEVEL

AVERAGE MAX VALUE

STAGEx_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

STAGEx_OFFSET_LOW

NEG ADAPTIVE THRESHOLD LEVEL = 39.08%

SENSOR CONTACTED

BY LARGE FINGER

STAGEx_OFFSET_HIGH

IS UPDATED

STAGEx_OFFSET_HIGH

IS UPDATED HERE

STAGEx_OFFSET_LOW

IS UPDATED HERE

STAGEx_OFFSET_LOW

IS UPDATED HERE

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

AD7148 Datasheet(PDF) 24 Page - Analog Devices

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