AD7147 datasheet(28/73 Pages) AD

28 / 73 page

Data Sheet

AD7147

Rev. D | Page 27 of 72

ADAPTIVE THRESHOLD AND SENSITIVITY

The AD7147 provides an on-chip, self-adjusting adaptive

threshold and sensitivity algorithm. This algorithm continu-

ously monitors the output levels of each sensor and automatically

rescales the threshold levels in proportion to the sensor area

covered by the user. As a result, the AD7147 maintains optimal

threshold and sensitivity levels for all 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 before a valid sensor contact can occur. The sensitivity

level is defined as how sensitive the sensor must be before a

valid contact can be registered.

Figure 38 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 and are based on the threshold sensitivity settings and

the ambient value. After the AD7147 is configured, initial

estimates are supplied for both STAGEx_OFFSET_HIGH

and STAGEx_OFFSET_LOW, and then the calibration engine

automatically adjusts the STAGEx_HIGH_THRESHOLD and

STAGEx_LOW_THRESHOLD values for sensor response.

The AD7147 tracks the average maximum and minimum values

measured from each sensor. These values provide an indication

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

results in a large average maximum or minimum value, whereas

a small finger results in 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 39 shows how the minimum and

maximum sensor responses are tracked by the on-chip logic.

Reference A in Figure 38 shows a less sensitive threshold level

for a user with small fingers and demonstrates 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 bit values and by the most recent average maxi-

mum 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 38

also provides a similar example for the negative threshold level,

with NEG_THRESHOLD_SENSITIVITY = 0011.

AMBIENT LEVEL

AVERAGE MAXIMUM VALUE

STAGEx_OFFSET_HIGH

25%

95.32%

62.51% =

POS_

THRESHOLD

_SENSITIVITY

25%

62.51% =

POS_

THRESHOLD

_SENSITIVITY

95.32%

NEG_THRESHOLD_SENSITIVITY = 39.08%

25%

95.32%

25%

95.32%

SENSOR CONTACTED

BY SMALL FINGER

AVERAGE MAXIMUM VALUE

STAGEx_OFFSET_LOW

NEG_THRESHOLD_SENSITIVITY = 39.08%

SENSOR CONTACTED

BY LARGE FINGER

STAGEx_OFFSET_HIGH

IS UPDATED

STAGEx_OFFSET_HIGH

IS UPDATED

STAGEx_OFFSET_LOW

IS UPDATED

STAGEx_OFFSET_LOW

IS UPDATED

Figure 38. Example of Threshold Sensitivity (POS_THRESHOLD_SENSITIVITY = 1000, NEG_THRESHOLD_SENSITIVITY = 0011)

AD7147 Datasheet(PDF) 28 Page - Analog Devices

AD7147 Datasheet(HTML) 28 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

Datasheet Download

Link URL