AD7151BRMZ View Datasheet(PDF) - Analog Devices
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AD7151BRMZ Datasheet PDF : 28 Pages
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COMPARATOR AND THRESHOLD MODES
The AD7151 comparator and its threshold 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 AD7151 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 23 and Figure 24).
POSITIVE CHANGE
POSITIVE
THRESHOLD
INPUT
CAPACITANCE
OUTPUT ACTIVE
OUTPUT
TIME
Figure 23. Positive Threshold Mode
Indicates Positive Change in Input Capacitance
INPUT
CAPACITANCE
NEGATIVE
THRESHOLD
NEGATIVE CHANGE
OUTPUT ACTIVE
OUTPUT
TIME
Figure 24. Negative Threshold Mode
Indicates Negative Change in Input Capacitance
Additionally, for the adaptive mode only, the comparator can
work as window comparator, indicating input either inside or
outside a selected sensitivity band (see Figure 25 and Figure 26).
POSITIVE
THRESHOLD
INPUT CAPACITANCE
NEGATIVE
THRESHOLD
INPUT INSIDE THRESHOLD WINDOW
OUTPUT ACTIVE
OUTPUT
TIME
Figure 25. In-Window (Adaptive) Threshold Mode
AD7151
INPUT OUTSIDE THRESHOLD WINDOW
POSITIVE
THRESHOLD
INPUT CAPACITANCE
NEGATIVE
THRESHOLD
OUTPUT ACTIVE
OUTPUT
TIME
Figure 26. 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 27).
FAST CHANGE
SLOW CHANGE
INPUT CAPACITANCE
THRESHOLD
OUTPUT ACTIVE
OUTPUT
TIME
Figure 27. 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:
Average(N ) = Average(0) + Change(1 − eN / TimeConst )
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 register.
See Figure 28 and the Register Descriptions section.
INPUT CAPACITANCE
(CDC DATA) CHANGE
DATA AVERAGE RESPONSE
TIME
Figure 28. Data Average Response to Data Step Change
Rev. 0 | Page 11 of 28
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