AD7150
Rev. 0 | Page 11 of 28
COMPARATOR AND THRESHOLD MODES
The AD7150 comparators and their thresholds can be
programmed to operate in several different modes. In an
adaptive mode, the threshold is dynamically adjusted and the
comparator output indicates fast changes and ignores slow
changes in the input (sensor) capacitance. Alternatively, the
threshold can be programmed as a constant (fixed) value, and
the output then indicates any change in the input capacitance
that crosses the defined fixed threshold.
The AD7150 logic output (active high) indicates either a positive or
a negative change in the input capacitance, in both adaptive and
fixed threshold modes (see
Figure 25 and Figure 26).
POSITIVE
THRESHOLD
INPUT
CAPACITANCE
OUTPUT
OUTPUT ACTIVE
TIME
POSITIVE CHANGE
06517-033
Figure 25. Positive Threshold Mode
Indicates Positive Change in Input Capacitance
NEGATIVE
THRESHOLD
INPUT
PACITANCE
OUTPUT
OUTPUT ACTIVE
TIME
NEG
TIVE CHANGE
06517-034
Figure 26. Negative Threshold Mode
Indicates Negative Change in Input Capacitance
Additionally, for the adaptive mode only, the comparators can
work as window comparators, indicating input either inside or
outside a selected sensitivity band (see
Figure 27 and Figure 28).
POSITIVE
THRESHOLD
NEGATIVE
THRESHOLD
INPUT C
PACITANCE
OUTPUT
OUTPUT ACTIVE
TIME
INPUT INSIDE THRESHOLD WINDOW
06517-035
Figure 27. In-Window (Adaptive) Threshold Mode
POSITIVE
THRESHOLD
NEGATIVE
THRESHOLD
INPUT C
PACITANCE
OUTPUT
OUTPUT ACTIVE
TIME
INPUT OUTSIDE THRESHOLD WINDOW
06517-036
Figure 28. Out-Window (Adaptive) Threshold Mode
ADAPTIVE THRESHOLD
In an adaptive mode, the thresholds are dynamically adjusted,
ensuring indication of fast changes (for example, an object
moving close to a capacitive proximity sensor) and eliminating
slow changes in the input (sensor) capacitance, usually caused
by environment changes such as humidity or temperature or
changes in the sensor dielectric material over time (see
Figure 29).
THRESHOLD
INPUT C
PACITANCE
OUTPUT
OUTPUT ACTIVE
TIME
FAST CHANGE
SLOW CHANGE
06517-037
Figure 29. Adaptive Threshold
Indicates Fast Changes and Eliminates Slow Changes in Input Capacitance
DATA AVERAGE
The adaptive threshold algorithm is based on an average calculated
from previous CDC output data. The response of the average to an
input capacitance step change (more exactly, response to the change
in the CDC output data) is an exponential settling curve, which can
be characterized by the following equation:
)1()0()(
/TimeConstN
eChangeAverageNAverage −+=
where:
Average(N) is the value of average N complete CDC conversion
cycles after a step change on the input.
Average(0) is the value before the step change.
TimeConst can be selected in the range between 2 and 65,536, in
steps of power of 2, by programming the ThrSettling bits in the
setup registers.
See
Figure 30 and the Register Descriptions section.
INPUT C
P
CIT
NCE
(CDC DATA) CHANGE
DATA AVERAGE RESPONSE
TIME
06517-038
Figure 30. Data Average Response to Data Step Change
