AD7147ACPZ-REEL Analog Devices Inc, AD7147ACPZ-REEL Datasheet - Page 26

• Manufacturer Part Number: AD7147ACPZ-REEL
• Description: IC,Converter, Other/Special/Miscellaneous,LLCC,24PIN
• Manufacturer: Analog Devices Inc
• Series: CapTouch™r
• Type: Capacitive Sensor Controllerr

Specifications of AD7147ACPZ-REEL

• Resolution (bits): 16 b
• Data Interface: Serial, SPI™
• Voltage Supply Source: Single Supply
• Voltage - Supply: 2.6 V ~ 3.6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-LFCSP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7147EBZ - BOARD EVAL FOR AD7147ACPZEVAL-AD7147-1EBZ - BOARD EVAL FOR AD7147ACPZ-1
• Sampling Rate (per Second): -
• Lead Free Status / Rohs Status: Compliant

AD7147
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
STAGEx_OFFSET_HIGH
STAGEx_OFFSET_LOW
POS_ THRESHOLD
AMBIENT LEVEL
_SENSITIVITY
62.51% =
NEG_THRESHOLD_SENSITIVITY = 39.08%
Figure 38. Example of Threshold Sensitivity (POS_THRESHOLD_SENSITIVITY = 1000, NEG_THRESHOLD_SENSITIVITY = 0011)
95.32%
25%
B
A
AVERAGE MAXIMUM VALUE
SENSOR CONTACTED
BY SMALL FINGER
95.32%
25%
STAGEx_OFFSET_HIGH
IS UPDATED
Rev. B | Page 26 of 72
POS_ THRESHOLD
NEG_THRESHOLD_SENSITIVITY = 39.08%
STAGEx_OFFSET_LOW
IS UPDATED
_SENSITIVITY
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.
62.51% =
AVERAGE MAXIMUM VALUE
95.32%
25%
SENSOR CONTACTED
95.32%
BY LARGE FINGER
25%
STAGEx_OFFSET_HIGH
IS UPDATED
STAGEx_OFFSET_LOW
IS UPDATED