Datasheet

AD7142
Rev. A | Page 24 of 72
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
CDC OUTPUT CODES
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
A
V
ERAGE MAX
V
A
LUE
STAGE_OFFSET_LOW
NEG ADAPTIVE THRESHOLD LEVEL = 39.08%
SENSOR CONTACTED
BY LARGE FINGER
05702-025
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