AD7147A Analog Devices, AD7147A Datasheet - Page 25

• Manufacturer Part Number: AD7147A
• Manufacturer: Analog Devices

Specifications of AD7147A

• Resolution (bits): 16bit
• # Chan: 13
• Sample Rate: 111SPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: Capacitive
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

ADAPTIVE THRESHOLD AND SENSITIVITY
The AD7147A 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 AD7147A 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 37 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 AD7147A 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 AD7147A 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
STAGEx_OFFSET_HIGH
STAGEx_OFFSET_LOW
POS_ THRESHOLD
AMBIENT LEVEL
_SENSITIVITY
62.51% =
NEG_THRESHOLD_SENSITIVITY = 39.08%
Figure 37. 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 25 of 68
POS_ THRESHOLD
NEG_THRESHOLD_SENSITIVITY = 39.08%
STAGEx_OFFSET_LOW
IS UPDATED
_SENSITIVITY
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 38 shows how the
minimum and maximum sensor responses are tracked by the
on-chip logic.
Reference A in Figure 37 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 sensi-
tivity percentage setting, the easier it is to trigger a sensor
activation. Reference B shows that the positive adaptive threshold
level is set at almost midsensitivity with a 62.51% threshold
level by setting POS_THRESHOLD_ SENSITIVITY = 1000.
Figure 37 also provides a similar example for the negative thre-
shold 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
AD7147A