Datasheet

AD7156
Rev. 0 | Page 12 of 28
COMPARATOR AND THRESHOLD MODES
The AD7156 comparators and their thresholds can be
programmed to operate in two modes: fixed and adaptive
threshold 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 AD7156 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
07726-033
Figure 25. Positive Threshold Mode
Indicates Positive Change in Input Capacitance
NEGATIVE
THRESHOLD
INPUT
CA
PACITANCE
OUTPUT
OUTPUT ACTIVE
TIME
NEG
A
TIVE CHANGE
07726-034
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 CAPACITANCE
OUTPUT
OUTPUT ACTIVE
TIME
INPUT INSIDE THRESHOLD WINDOW
07726-035
Figure 27. In-Window (Adaptive) Threshold Mode
POSITIVE
THRESHOLD
NEGATIVE
THRESHOLD
INPUT CAPACITANCE
OUTPUT
OUTPUT ACTIVE
TIME
INPUT OUTSIDE THRESHOLD WINDOW
07726-036
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
A
PACITANCE
OUTPUT
OUTPUT ACTIVE
TIME
F
A
ST CHANGE
SLOW CHANGE
07726-037
Figure 29. Adaptive Threshold Indicates Fast Changes and Eliminates Slow
Changes in Input Capacitance
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 30). The sensitivity
value is programmable in the range of 0 LSB to 255 LSB of the
12-bit CDC converter (see the Register Descriptions section).
NEGATIVE
THRESHOLD
POSITIVE
THRESHOLD
DATA AVERAGE
OUTPUT ACTIVE
TIME
SENSITIVITY
D
A
T
A
SENSITIVITY
0
7726-039
Figure 30. Threshold Sensitivity