Datasheet

AD7151
Rev. 0 | Page 12 of 28
SENSITIVITY
In adaptive threshold mode, the output comparator threshold is
set as a defined distance (sensitivity) above the data average,
below the data average, or both, depending on the selected
threshold mode of operation (see
Figure 29). The sensitivity
value is programmable in the range 0 to 255 LSBs of the 12-bit
CDC converter (see the
Register Descriptions section).
NEGATIVE
THRESHOLD
POSITIVE
THRESHOLD
DATA AVERAGE
OUTPUT ACTIVE
TIME
SENSITIVITY
DAT
A
SENSITIVITY
0
7086-019
Figure 29. Threshold Sensitivity
HYSTERESIS
In adaptive threshold mode, the comparator features hysteresis.
The hysteresis is fixed to one-fourth of the threshold sensitivity
and can be programmed on or off. The comparator does not
have hysteresis in the fixed threshold mode.
OUTPUT
POSITIVE
THRESHOLD
DATA AVERAGE
OUTPUT ACTIVE
TIME
HYSTERSIS
DAT
A
07086-020
Figure 30. Threshold Hysteresis
TIMEOUT
In the case of a large, long change in the capacitive input, when
the data average adapting to a new condition may take too long,
a timeout can be set.
The timeout becomes active (counting) when the CDC data
goes outside the band of data average ± sensitivity. When the
timeout elapses (a defined number of CDC conversions is
counted), the data average (and thus the threshold) is forced to
follow the new CDC data value immediately (see
Figure 31).
The timeout can be set independently for approaching (for change
in data toward the threshold) and for receding (for change in data
away from the threshold). See
Figure 32, Figure 33, and the
Register Descriptions section.
DATA AVERAGE
+ SENSITIVITY
LARGE CHANGE IN DATA
DATA AVERAGE
DATA AVERAGE
– SENSITIVITY
TIME
TIMEOUT
07086-021
Figure 31. Threshold Timeout After a Large Change in CDC Data
INPUT CAPACITANCE
THRESHOLD
OUTPUT
DATA AVERAGE
OUTPUT ACTIVE
TIME
TIMEOUT APPROACHING
07086-022
Figure 32. Approaching Timeout in Negative Threshold Mode
Shortens False Output Trigger
INPUT
CAPACITANCE
LARGE CHANGE
THRESHOLD
OUTPUT
OUTPUT ACTIVE
TIME
TIMEOUT RECEDIN
G
07086-023
Figure 33. Positive Timeout in Negative Threshold Mode
Shortens Period of Missing Output Trigger